Both games are very short in the single player department (I finished both in 6-8 hours). Of the two however, I'd say MW3 was the best of the two experiences, but only if you are up-to-date on the previous two MW titles and like it's Hyper-Realism.
Don't get me wrong, BF3's campaign was very good, especially from the more authentic stand-point. Thing is, I just had a lot more fun with the campaign of MW3, plus it has Spec-Ops and Survival mode.
In reality, I wouldn't recommend either title. They both are short and not quite worth a full $60 for just the price. Now, if he likes a Sci-Fi twist with his Military Shooters, I think the above mention of the Crysis series is actually a very good suggestion. Those campaigns (Crysis 1 & 2) are longer and have a better story (Subjective, I know).
As far as a strictly modern military shooter goes, however, I'd actually be the crazy guy here and suggest Medal of Honor (The modern reboot). You can get it cheap in comparison, and I actually was more impressed with its campaign than CoD or BF3. It was a little bit longer, and it actually did have a rather authentic feel that was executed better than BF3 (IMHO).
In the end though, it's your dad, so it's up to you.