@Nic74
Let us know how it turns out
This is bang on! @whero locks onto the key consideration of how he plans on using it into the future.Welcome to TalkBass.
You're an awesome mum! My father (who was a professional musician himself) bought me my first bass and amp/cabinet back in 1971 through his friends and contacts in the music retail business. If I'm honest, it was awful gear, but I understood he didn't have a lot of money to start with and he wasn't going to spend much of it on something I might have given up after a week. It was loud enough to play in my bedroom (and annoy my mother) while I was learning but I needed to upgrade pretty much as soon as I started playing with other people. I was so happy to get it, and it set me on a path I'm still on today.
Now, I know nothing about your son, but you'll probably know whether what I say next sounds like him or not: I'm sure he will appreciate what you're doing for him, and be grateful. The difference between your son and 1971 me is that you say he's picky because he's into sound already. I wasn't picky back then because I knew nothing about anything. I just loved listening to music and wanted to try making it myself. If he knows what he wants, there's a chance you could disappoint him if you don't get "the right thing".
As some others have said, the best thing might be to engage him in the process. If he talks to you about his music, ask him questions. What does he want to achieve with his music? You say he's into "sound engineering" - does he want to perform live music with a band, or is he more into mixing and recording? If he's into recording, he may not need something that produces a lot of volume. If he's going to play in a band, it probably won't be long before a 40W Fender is not enough.
Personally, I like the idea of you taking your son to the shop (particularly if you think you can do it as a surprise), letting him try some different things, and choosing something, rather than giving him cash or vouchers. I'm sure he'll appreciate you being there, taking an interest and supporting him.
Let us know how it turns out