Some kids are just a bit different. :) But if your son can lift his head when he is on his stomach, he'll be just fine. You can use toys or food to persuade him, but... extension (stomach to back) develops before flexion (back to front)... so if your son has reached all or most of his developmental milestones, there shouldn't be any concerns. Around 5 months, he should be able to sit with using his arms (palms down for support). Crawling happens about 8 months, cruising along furniture at 11 months, walking at 14 months... and these are just estimates. Children are either shakers or talkers first, so if your son is chatty, his physical develop might be a bit off or delayed. And this is normal. And some children skip some steps. If for some reason your son doesn't sit with arm support at 5 months, or starts rolling soon after that mark, then you might want to follow-up with the pediatrician. If your pediatrician feels there's a serious developmental delay, your son should be consulted to a pediatric physical therapist or occupational therapist. I think your son will be just fine. :) :)