So scared about my 18month old son

hi everyone, first post so looking some much needed advice (and encouragement).

We have a lovely 18month old son who (we think) is displaying some worrying traits. He was a 4lb premature baby but didnt need any special care and thrived. He walked early and started babbling early. He sleeps great and eats near enough anything. He started saying mama and dad at about 13 months. Other small words followed like banana, light, apple etc. The last couple of months we have noticed he doesnt talk as much but still says the odd word (ball, dad etc). He climbs and is very active - loves going for walks. He doesnt really interact with other kids, he would smile at them sometimes but isnt interested in playing with them. He is an only child. He will only play with toys a bit (cars, ball). The worrying things he dors is he rolls everything between his hands (from an early age) but we didnt think much of this. Recently he has started looking out the sides of his eyes a few times a day (usually when hes tired). occasionally he would flick his fingers too. He does give eye contact and smiles a lot at us. He can point when he wants something. When I google things it has worried me sick that his development is slow and he should be talking more. we brought him to the GP who was pretty useless - no real interest in talking about what milestones he has hit "he is still a baby, come back in 6 months if you're still worried". We also visited our HV who says his general health is great but he should be saying more words. She has referred him for an assessment. Being the impatient person i am I have enquired about private assessments and a private diagnosis as I have read not to leave it until 3 or 4 for a diagnosis and help, intervention is needed now if something is wrong. We have previous bad experience with fertility with the NHS and their "just wait it will be fine" attitude.

We are breaking our hearts that our little boy might be affected but of course he will be loved the same. We just want the best for him. Would really appreciate any advice.