I had a similar situation several years ago, when my niece, who lives in England, was given a cheque from an Irish bank account. We were told to take it to the issuing branch to be cashed, which we were (luckily) in a position to do.
Alternatively, you may be able to lodge it to your own account. Check with your bank to find out for sure.
This might be the ideal opportunity for you to open a bank or post office account for your son. Otherwise just get him to scrawl his name on the back of it, or just sign his name, give him the cash & lodge it to your own bank account.
our local credit union took daughter's cheque just I had to sign my name on back after hers.
Decided recently to open up a PO account for her and it was the best thing we did as now she gets pocket money on a friday and manages her own money and small purchases. very good learning for her, although she is nine years of age.