Don’t fear the assessment and the potential diagnosis, OP. They are looking at what is different with your child and if they find him to be delayed, they will tell you as much. If they find there is another better name for how his brain behaves, they will tell you that. No matter what they tell you, it gives you and his school an understanding of what support is best for him and that is the win you need!
Do you know the expression ‘it takes a village to raise a child’? Right now you need your village because a child with a language delay needs to be exposed to appropriate rich language opportunities. That means human speech, not recorded speech, as much as possible. Once every six weeks is only enough to observe any steps backward - it’s not enough to make forward progress. If he can’t speak back to you, that doesn’t mean you speak with him less. If anything, you narrate to him as much as possible. He needs to hear and see people talking and there needs to be pauses that would allow him to respond as a more fluent child would, even though this will
just be silence right now. So time to get your village in line: make an appointment to speak with the class teacher and one with the senco. The SALT would be great too if possible. Get your head into the mindset of ‘how can I help my child improve his speech?’ so you’re out of the mindset of ‘what if my child has autism?’ Because one of those is far more practical and helpful than the other.
Once you have met with them to have their support of what can be done at home, then look at this side of your village. Who spends the most time with your child? If there are times when he has to sit without human interaction, who else can you speak so you can get more hands on deck? How often does anyone read picture books with him? How often does anyone narrate his activity to him, like now we are getting dressed. This is your shirt, these are the buttons. Let’s find your socks. You have two feet so you need two socks. One, two. Two socks! And so forth.
If it is just a speech delay and things have been physically checked that everything in his mouth works etc (remember older children can still have a tongue tie, not just babies!) then a real effort to surround him with enriched language opportunities for a good six months should give you and his teachers a sign of what progress can be made. It is worth the effort!
Above all, don’t use your energy for worry right now. Use it for practical ways to help your son develop key skills.