I had/have this same issue. I find that if I take time to play with my DS and he starts throughing a fit, I just tell him that he has to straighten up or lose time with me and I leave him to think about it.
Are his "fits" getting a rise out of you? What I mean is are you giving him more attention in dealing with them? What worked for me was to absolutely ignore the tantrums and them would stop almost immediately.
Remember all things take time and transitions are hard for little ones. Don't give him a reaction and they will soon stop.
I commend you for knowing that even though you feel sorry for him, rewarding this behavior will only perpetuate (sp?) the situation.
Hope I helped!
Sheila