11 week old barely lifting head

My son is 11 weeks. We do tummy time several times daily. On my chest and on his boppy mostly. He will lift his head on my chest more than anywhere else. The boppy a little bit. But flat on the floor he doesn’t at all. He just lays his head down and cries. He HATES tummy time. I’ve been doing other things to try and strengthen his muscles in his neck that I saw on YouTube as well but still he refuses to lift his head flat on the floor.

At the doctor this is how they test his strength. They put him on his belly on the bed and he doesn’t lift his head and they told me he’s behind. I’ve been panicking about it so I’ve been doing it 3-5 times everyday plus the other  exercises but it’s been 2-3 weeks and I see no difference. Hell only hold his head up for like 5 seconds on the boppy then it’ll drop.

I feel like I’m failing him and I’m so stressed about it. What else can I do?