There is really nothing you can do - from an action standpoint. When I was small I did the same thing with my mom and dad and the fact was - I just felt more comfortable around my mom. Of course, I loved my dad - but I wanted to be with my mom, period. Just being open minded with him and understanding where he's coming from - you know - he's with her all the time and then comes with his dad and there is seperation anxiety and all that going on. He's also just 2 1/2 which is very young. He may not grow out of it for a while. The best thing you can do is get your energy as high and you can before he gets there so the energy feels good to him.