There are heavy handed approaches if necessary, telling him that what he is doing is off-camera, telling him to stop, and kicking him out of the group are last case scenarios, although before that, I would say that you should talk to him about changing his character into something more group-friendly. However the game will probably be better if you take other more subtle approaches
First and foremost, there are levels for a reason. There are plenty of monsters that are designed to be taken on by a a party of 4 varied classes. All you have to do is allow the player to go off on there own, and encounter a perfectly reasonable creature from your world that he can't handle alone. Is he playing a wizard? Well throw in some decent level Drow rouges with sneak attack and a bunch of attacks of opportunity and significant magical resistances. Is he a bruising barbarian? Find something with a lot of charm, stun, and illusion, or magical defenses like Stoneskin and mirror image. A sneaky rouge might run into something with tremorsense to negate the sneakiness, and a bunch of things immune to critical hits. Or perhaps another party member has a silver/ghosttouch/adamantine/whatever weapon, and the lone wolf realizes that hes stumbled into some werewolfs/ghosts/golems/whatever. To a large extent, even just throwing in a monster that's a reasonable danger to a party of 4 should be able to present an impossible task to a lone character, especially once you factor in those flanking bonuses. The challenge doesn't have to be limited to combat either. Make sure you include a lot of deadly, crippling traps that a rouge needs to disable, that delivers poison that must be cured magically by a cleric, a key plot item beneath a church that will only let a party in if they get the testimony of a adventurer who worships that God, Arcane writing leading to an item that can only be unlocked by magical means, tasks that must be performed by a certain race, favores needed from another characters patron, challenges that require certain skills...the possibilities are endless. Just 1 locked door that needs to be picked and then opened by channeling divine power will stop a lone wolf player cold, and if the big bad boss's massive treasure vault is behind that door...
You could also introduce a little linearity. Variety is the spice of life, so if you have been doing a big sandboxy game, make the next major plot point be in a liniar dungeon crawl. Theres really no way to go off to the side when the entire quest is backwards and forwards. Maybe he will get used to functioning as a fellow party member, and the problem will dissipate.
Also try going with the flow. Just once, throw in a plot point where some baddie group are invoking racist rhetoric and subverting his species, and the party has to oppose them, both to help the discriminated against party, and further their main goals. Let the party get all friendly with the lone wolf characters race at large, and just wait for them to jump on the character-growth plot ball. Maybe the character sees a fellow party member save a child of his race when he wasn't able to, and he decides that maybe all non-his species arn't that bad. Or give rewards for cooperation. Maybe he wants to multiclass and needs a mentor. Or maybe his leaders want him to learn about other races, or want him to help with the parties goals so the other party members patrons owe his patrons a very important favor, and failing to help leads to a serious dressing down by his superiors, and serious repercussions for the people he cares about. War makes for strange bedfellow, and failure to make those enemies into friends can lead to disaster. Makes for some good intrigue too.
You can also give him some subtle hints. Give the character his side-quest. Maybe he defeats a monster or two, but because he went away from the main party, his side quest involves a lot of waiting and traveling. Then go back to the main party, and run your quest. Give them most of the spotlight, loot, and experience. If this guy is so dedicated to roleplaying his character realistically, he won't be able to complain that the adventuring that the main party is doing in the most important areas that have the most information about terrible things happening and that the rest of the party thinks is the first priority happen to also have the most rewards. He will change his tune pretty quickly.
If nothing works, then just talk to the player. Most overly focused players think that they subscribe to the idea that fun is the only thing that matters, even when they really don't. Tell them outright that there character is making the game a lot less fun. Don't blame him for it or point any fingers, just tell him the truth, and try to figure out a way around it.
Basically, make his racism and desire to be alone interesting to the plot. Don't try to force him to act like something he is not, make the character decide that rationally he has to put aside his objections for a larger goal. That's the best way to eliminate racism, even in real life. Concoct scenarios where he needs a party, and make him believe that it is his characters idea for him to do so, not yours. If you need other ideas, just think of fiction where a person is a loner or a bigot, and he is forced to work alongside those he dislikes for a more important goal.
This is all things I can gather based on general RPG knowledge. If you could tell us the system, the setting, and a little background like the classes in the party, other parties personal goals, and significant NPCs, we could probably help you a lot more. I'm kinda assuming D+D because it seems the most likely.