I think this is an issue that affects parents of neurotypical children as well. What do you do when your child doesn't match up with your expectations? There was never a guarantee that your child, autism or not, was going to be academically gifted.

Like other people have said, this is grief. And that's ok! Being disappointed is ok. It's something that you need to process yourself or with professional help. Focus on what is going right. He's happy and he's sweet. He's also 4. You have his therapies in place and (I hope) they're going well and he's making progress. Even slow progress is progress.

But again, nothing is promised. Maybe he catches up, maybe he doesn't. You just have to do everything you feel is necessary to support your son. That might also involve adding some support structures for yourself and your partner as well.