First off I would get him seen by a Doctor as any concussion can be serious and the good news is the NHS take the same view. Given he has been sick some time after he might have taken more of an impact than you think.
As for when to get back. I would say three weeks is about right. I went over the bars a month or so back taking a good blow to the head. 3 weeks seems like about the time it took be to being back feeling 100% right. As you say diving when done right is safe, but if he is not functioning at 100% then the risk of a repeated head injury goes up as he is more likely to make a mistake.
Hope this helps and Junior is feeling better.