Here's a sort-of how-to on paying with bitcoin. I'll be happy to give you a detailed step-by-step guide for it if you want. But it's fairly simple and you should be able to figure it out on your own.
Just signing up for the Cash app doesn't enable you to buy or send bitcoin. To get set up for that is a separate process from the one you go through when you first install the app. If you haven't done it yet, it involves taking a picture of your driver's license. Banking laws require Cash to get that from you before you can buy bitcoin. Don't get paranoid about it unless you're intending to do a little money-laundering.
If you've already done all that, then the first thing you'll have to do is buy some bitcoin using the Cash app, a simple, more or less self-explanatory process. . Once you buy the bitcoin, you turn around and send it to him.
As others have noted, the value of bitcoin has been so volatile lately it's recommended that you send payment immediately after buying it rather than sitting on the bitcoin for a few hours or days before you send it. It's entirely possible that the value of what you purchased could drop below the amount you need to send. I always include a few bucks extra because there are transaction costs--at most a few bucks, a trivial amount--and you want to make sure you allow for that so you don't send insufficient payment.
When he requests payment, he'll give you something called a bitcoin address. All you have to do is select "send" in the cash app, specify the amount, then put that bitcoin address in the blank for recipient. The address itself is just a long alphanumeric code of like fifteen or twenty characters.
As noted before, after your first order, when you order again don't use the same bitcoin address. He'll probably give a new one for every order.
Once the transaction is final, Cash will send confirmation to whatever email address you gave when you signed up for it.
Speak up if you want more details than that, or if you have any more questions.
