I moved to Sydenham 1 year ago (18th October). Unless I am forced to I can't imagine I will be moving anywhere else soon. The high street has everything I need, transport links are fantastic and the people/community are great. I moved here from Bethnal Green so I do know what you mean about fitting in and I have never felt so "fit in" as I do now living here.
2 Fantastic doctors surgeries with an array of specialist clinics. A really good dentist on Lawrie Park Rd (which is mandatory for me as I am dentistphobic!)
Also, I think Pally above means Forest Hill Boys, probably an auto correct on his/her phone

I have a friend who works in the SEN field, albeit in Ashford, but if you have trouble finding a good fit for you son, I am sure she will have a database or something that I can get you a link to of decent schools for children with additional needs.
Additionally, there are some other forum topics on the subject of moving to Sydenham that you may want to get some feedback from also.
https://sydenham.org.uk/forum/viewtopic.php?f=8&t=13413
https://sydenham.org.uk/forum/viewtopic.php?f=1&t=13705
https://sydenham.org.uk/forum/viewtopic.php?f=8&t=13619
https://sydenham.org.uk/forum/viewtopic.php?f=8&t=13571