Autistic son has no photo ID and will be migrating to universal credit

There will be other ways to verify his identity, such as his debit card.

Which benefits is he claiming?

Does his ESA include the Severe disability Premium? If it doesn't and he's in the Support Group for ESA then UC with the LCWRA element pays more than ESA Support Group.

If he doesn't have photo ID then he will need 3 pieces of non-photo ID for the verification check.

Debit card is one of those. Printed bank statement seems to be counted separately so that makes two. Then I was suggested a tax letter or similar for three. I do get a tax letter every year, even though I don't pay tax, so I guess everyone else does as well. (I think birth certificate counts as one of the three as well).

He will also need something with his current address on it. Printed bank statement can be used for that, although I'm yet to see whether they'll allow that to be used again after it's been used for the section above.

He will also need proof of his National Insurance number. Depending on age that could be on a card or just a piece of paper.

He will need a bank card for the account where the UC is to be paid.

And if claiming housing element, will need proof of rent costs as well. (Not sure that applies in this case after reading the first post)

PIP & ESA letters can't be used unfortunately. It needs to be something from outside of the DWP.

If you can get a bank statement printed, that should be good enough for proof of address.

I know it wasn't asked about in the OP but they do seem to need to 'see' the claimant now, even in cases where they have an appointee. So the ID check is generally done at a Job Centre. If that's not possible for your son, they can arrange a home visit instead. If neither of those are possible, that's where it gets more difficult.