Hey there, @Stabben89!
Thanks for coming to the Spotify Community.
We're here to help, don't worry!
Firstly, can you check:
1). The steps here on Premium for Family.
2). Has he opened a Spotify account under this email yet? If not, can he try opening a Spotify account first, and then re-send the invite?
3). Are you able to try sending the invite to a different email?
4). We know it sounds simple, but we'd also recommend double-checking any junk/spam folders too, just in case.
If these tips don't help, get right back to us 🙂
We're happy to look at things more with you.
All the best,