I think in the long run he does need a coach. As good as he is the game will keep evolving, and of course he has the ability to evolve with it. But seeing things from the perspective of another person alot of the times can help and sometimes it is needed even for a guy as brilliant as Federer is.
Strategies and game-wise aside, a coach is also there to organise a player's schedule- be his intermediatery. With Federer's packed schedule it might be a good idea to have someone help him out with it as doing it all by yourself is very demanding and wearing.