Claim DLA! Yes it's trickier to claim with a baby/toddler but not impossible... google the Cerebra guide to help, because it will really help you list all the things you do above and beyond what a 'typical' toddler would need... I'm guessing he can't self feed, needs repositioning constantly, needs a ton of extra stimulation because he can't access the world by himself... needs more than average help at night... see where I am going?
Oh and if turned down..wait a few months and apply again! We were turned down first time but not since!
Physio is useful of course but not the be all and end all tbh. If your eally have to wait another 4 months , start making a list now.. of what you need and want to happen next;
Ie a referral to Paediatric OT, referral to wheelchair/seating services (may come under the same umbrella) Dietitian if he isn't safely eating solids, and definitely genetics!
Sometimes there aren't answers... I work in special school (my son went there and I'm still there long after he has grown up!) and there are many children who never get a firm diagnosis... but others do, even if it takes years, or a random 'let's just test for this' from a geneticist.
There will be battles ahead in many areas... not trying to freak you out, but it's a fact.. so if you can get your ' my child's needs WILL be met' head on now, and prepare to be firmly demanding (aka a right pain the ass if necessary) it will all help your little lad be the best that he can be :)
My son didn't smile til he was one, he was this floppy wailing baby whom my Mum described as like a miserable little alien (he did have a very big, wobbly head!!!) but at some point between one and two he did at least stop wailing and became a sunny smiley boy...which made everything a lot easier to cope with!
Hang in there... start making lists, right now referrals and getting 'into the system' are one of the most important things you can do.
And ((hugs)) It's bloody hard. I remember the early years and wish I could go back now, and tell my old self that it would be ok..that I would be ok. My son has done far better than we ever imagined but he won't ever be independent... that thought broke me when he was tiny, now he is just a fab young man just as he is!