What thumb tips is he using?
Regardless, it's probably a Vernet.
There are multiple versions/sizes for TT.
Do you want a harder plastic one or, maybe, a softer type?
Determine the size based on your thumb (of course) and what types of tricks that you're planning on performing.
Candidly, I have an assortment of different TT. All will work for the vast majority of TT 'work'.
It's a good thing that they are inexpensive.
Although it's in every kid's beginner magic set and you'd think it might be 'overexposed', it still is a great little utility.