i would start by sitting on the floor with them and encourage positive play, have them give the ball and say thankyou, your welcome. Have them share, take turns, toss to one another...the more you role play with them the more they will pick up a more positive form of being together. Sounds like your son may be envious or bent out of shape over this guy in your home with his toys and his mom. Do you have his stuff seperate from daycare? what do you do with him when he acts this way? do you get after him, firm words? remove him or encourage a more productive response? when he bites and hits the other boy do you show concern for that boy in front of him and teach him it hurts?