You will need a written contract signed by both him and you. Print it, sign it, send it to him by snail mail, tell him to sign it, and have him send it back by snail mail.
Ask your lawyer to write such a contract.
When you don't want to use snail mail and stay completely electronic, ask your lawyer if it might work in your jurisdiction to print the contract, sign it and scan it with the signature.
You don't have a lawyer? Get one. The moment you start making business with 3rd parties you need one.
When you really want to avoid paying a lawyer (a decision which might bite you back later) you can still try to draft your own contract. Contracts are still valid when not written by legal professionals. But they will be treated as if they were, so you need to be really careful with what you write. When anyone decides to sue the other party, the contract will be interpreted word-for-word not by what you actually meant. Important information it requires:
- When, how, and how much do you pay him?
- Which song exactly do you get the rights to?
- What rights exactly?
- For what purposes?
- Just this project or any further projects?
- For how long?
- Exclusive / non-exclusive?
- Are you allowed to edit it?
- (important!) What happens when the artist doesn't actually have the full rights to the song and a 3rd party sues you which claims the song violates their rights?