My son just started playing wow and I want to link his account with my battle net account link I did my other sons account years ago. But I forget how.

Please head over to the WoW Customer Support forum. The folks there can help you out with how to deal with a minor child on a Bnet account. (hint - don’t put him on yours). If, instead, you are talking about the Refer a Friend promotion and benefits, they can explain that to you too.