Potty training issues

Hi everyone, I’m new to the site. 

I am having issues potty training my 3 year old, we are awaiting appointments for asd diagnosis, spd seems likely too. 

We originally introduced the potty at 2 and a half years old, absolutely petrified would not sit down or even so much as look at it. Since then we have introduced it to him almost every fortnight to try get him to come to terms with it.. he will now look and even sit on the potty. The problem is actually doing his business on the potty! He knows when he needs to go and we will encourage him to use it and help him in anyway he needs to feel comfortable but he will just not use the potty to actually do any 1s or 2s.

he starts reception in September and this is something we would ideally prefer to tackle before reception. I’m just really lost at what to do next and who to speak to regarding this.

thanks for reading x