What To Get An Autistic 12yr Old?

So it's my autistic son's 12th birthday on Sunday. I have not bought him a single present and I feel awful! He's a nightmare to buy for, and gifts are often a rushed, last-minute affair. But this is the closest yet that we've got to the big day with absolutely nothing and more worryingly, we still don't have any ideas! He gets highly focussed on one topic, but even this is proving difficult this year. He's a maths nerd, so we usually try to incorporate something along those lines. Short of getting him a stack of GCSE papers (which honestly he does enjoy!) then most maths books are either too easy or too wordy/boring! In the other direction, he enjoys low-effort toilet humour. The more gross, the better! He watches a lot of YouTube channels, but the ones he likes don't really offer any merch. It's a bit late to order anything from Kurzgesagt now, and he's already got their awesome Immune book. He does enjoy Roblox and only thing he has asked for is robux so he can buy some in-game…