tummy time help

Madi
Our son is 2 1/2 months old and he barely can lift his head while on his tummy. He can keep his head up while sitting up. We do tummy time a few times a day and he HATES it. What can I do? When should I be really concerned about him not being able to lift his head? He is trying really hard and is using his lower back muscles to try to get up