exactly what Nigel said, cash only. When I land here I always arrive with extra cash (it never hurts to have extra cash.) Looks like you're in the US. That said 20K THB = nearly $550 USD right now so I'd bring maybe $750 USD to allow for any exchange fluctuations.
a more correct statement would be "You probably won't be asked for proof of accommodation." Just like he probably won't be asked to show 20K THB in cash & proof of onward travel. However, it's always a possibility & I've seen numerous posts over the years in this group & other groups of people reaching out for help, because they've been denied entry for one of the reasons listed above.
If I were in your situation, I'd take time to read the fine print on the policy paperwork that they should have provided you. I'd imagine the whole "waiting period" so many people are speculating about would be written in the policy details.
The easiest way on a tourist visa is to use an agent. I hear good things about TikTok Services but I've never used them. You can search for them on FB and they will pop up but really any of the Visa Agents should be able to help you. Be aware that since Songkran begins tomorrow, visa agents will probably be closed until at least the 17th or maybe even the 22nd. Good luck and Happy Songkran!!
actually having thought about it, I would order from Power Buy. It is possible you could get a knock off at one of the other sites. If you go with Power Buy, you should get the genuine article (e.g. Asus, HP, etc.)
Honestly, you'd probably save some money if you buy him a new 12,000 Baht laptop on Lazada and just have it delivered to his house. A 12,000 Baht laptop will do any task a student needs. Also, Songkran sales are beginning now so you might get one for 10K vs 12K. Try Power Buy, Lazada or Shopee.