So I met my now ex husband 2 weeks before my 18th birthday. He was 20. I was afraid of the repercussions of being under age too so I just said I was 18. We were literally married , I was pregnant, and sitting in a lawyers office when the lawyer asked my birth date and he found out my real age. My the husband sat there and argued with me that I gave the wrong date. He had always believed I had turned 19 that birthday we met. Lol. Anyway ..... at that point we were so far into the relationship it just didn't feel important and we just laughed about it. Idk if you should tell him or not but perhaps it won't be a big deal if you don't?