I would give them an idea, if you don’t say they will still buy something anyway and it won’t be something you are going to use. Could you suggest a couple of broad options that cover a few price ranges such as some books by a certain author or food role play toys or push along cars etc
I always send a few things in varying prices some £10 and some up to £100 then they can choose what they would like to get! My grandparents bought us an annual pass for Warwick Castle which is something we can use all year. - Balance Bike - Toy Kitchen or accessories - threading beats - magnetic letters - clothes
Honestly I’d just be adamant for vouchers and money so you can use it when he needs things and for days out. Just say whatever they buy him will just be useless because he doesn’t need anything