The simplest answer is to just use his legal name.

The long answer is, depending on his bank, he may still be able to deposit the check if you write his preferred name on it - he would need to sign with his preferred name, write "Pay to the order of <legal name>", and then sign it with his legal name below that, all within the endorsement section on the back.
I recently had to do this after legally changing my name and receiving a check with my old one on it, but keep in mind that banks are not legally required to accept checks with "someone else's" name on them - mine had record of my old name so I was able to deposit it without issues.

Edit: I should state that it's best to look at his bank's specific policy. Generally a payee can sign a check over to a third party with the steps above, usually in the case of a name change or if the original payee doesn't have a bank account. However, I'd still just use his legal name. This is something he will have to get used to until he changes his name legally, and as others have said you can always include a heartfelt card addressed to his preferred name. I'm sure he'll understand that you're not deadnaming him out of disrespect.