Yes, He Should!
I personally think that he should be part of the band, because Italy is famous for their well-known tenors. So he is the perfect candidate, and I was very disappointed that he decided to leave in the first place!
He would be considered as the youngest member of the gang if he had not leave.
No, He Made the Right Decision
Since he was already well known prior to Il Divo, he would not be part of the band. If this was so, it would alienate many of his fans, and not to forget Il Divo's fã who grew a liking on the quartet themselves. So, he made the right decision!
Conclusion
If you think that he should be in the band, I don't mind at all!