Hiya sorry for late reply.
My daughters story is abit different, she was born at 31 weeks and a routine cranial ultrasound picked up significant brain damage at just two weeks old. From that moment we knew she was very high risk for having cerebral palsy, she wasn't officially diagnosed until about 18 months, but we knew for sure from about 9 months that she had some kind of CP. they really don't like diagnose it early unless it's severe and overwhelmingly obvious or like my daughter they have a brain scan which clearly shows brain damage linked with CP. this is because so many children do catch up! A diagnosis before age 2 is rare.
The bonus of us knowing very early on that she would most likely have CP was that we could start therapy straight away! She's had weekly physio since she was a couple of months. We now do weekly physio, hydro, brain plasticity programme as well as lots of at home therapy. Her predication from the doctors from her brain scan was severe quadriplegic CP, but she actually has moderate/mild spastic diplegia (affecting just her legs) - she's just starting to take steps with a kaye Walker at 30 months, And she's bright cognitively - talks better then some three year olds, can count to twenty, knows colours, shapes etc so think it's just a physical disability.
It's a really positive sign that he's social and meeting his cognitive and social milestones and possibly means his delays aren't linked to ASD (which can cause motor delays too).
Your GP really is failing you, I hate the 'wait and see' attitude when it's been proven that if something is wrong early intervention is key! It doesn't surprise me though, even with my DD having significant brain damage and being told she will most likely have CP there was still no proactive therapy given - we were just sent home and told to wait and see! We had to do it all privately until she was around 1 when NHS and children's services started to help more - but even now we have to pay for most of it, until they get to school age provision is just really poor in some areas.
I really hope the HV is more helpful and they don't keep you waiting. In the meantime you tube has lots of good paediatric physio videos you can watch. If he's not rolling yet start with some assisted rolling - start with his legs and get his body to follow through the movement. Side lieing and using hands to play whilst lieing on their side is good for building core strength too if he's not tolerating tummy time. Have you got a tummy time pillow too? He might prefer it with that. Swimming is also great!
Keep strong mumma, I promise you reality is never as bad as what you are envisaging in your head. Even if something isn't quite right, it won't change him, he's still your gorgeous little baby and you will both grow and adapt and it will be ok! Xxx