T.K.
No transition. Cold turkey. Start today to talk up the big boy aspect. Go to his room today and have him help choose his favorite sheets, pillow and something soft to sleep with. Praise him all day for being a big boy sleeping in his big boy bed. Start the bedtime routine a little early. After bath, take him in his own room and read to him in bed. Maybe give him one of your silky nightgowns or a pillow from your bed to cuddle with. It will smell like you. Have dad get in on the praise too. Tell him how proud he is of his big boy going to school and sleeping in his own bed. If you have to, tell him they wont let little boys that sleep with thier moms go to school, only big boys that sleep in thier own bed or they might not think he's ready for school. That will definatley motivate him. Be so very consistant. No backsliding on this! They will try you if they think there is any wiggle room. I've gotten my 5 yr old broken from it, but he still sneaks in if he's sick or if dads out of town, in the middle of the night. But thats because I let him. Dad is zero tolerance and he doesnt even try it with dad. BE FIRM!