Incarcerated and claiming dependents.

I was wondering, my sons father was incarcerated for 8 months in 2019, he's trying to claim our son whom I had for at least 6 months while he was in prison. I had custody of him and provided for him from the end of February until May 1st, and then had him all summer. His legal address was at his Grandparents house. If his father was incarcerated he wouldn't be able to claim him right? Because he didnt technically live with him for 6 months out of the year? Any input would help.