Husband just found out he has another child

My husband (41) and I (41F) and I have been together for 17 years. He just found out (from the child) that he has a 19 year old son.
LSS- They both did 23 and Me and discovered the connection. Mom was a one night stand while my husband was working travel construction in the summer during college. She says they met at a bar and didn’t exchange last names so she did not know how to find him.
We are both excited to meet his son and welcome him in to our lives. However, my husband is heartbroken that he missed his son’s entire childhood. He’s an amazing father to our daughter and has always wanted more children. Any ideas on ways I can help him through these emotions?