Baby is 15 weeks

And he still doesn’t lift his head all the way up on tummy time he does lift it but not push and extend his arms. I’m worried he’s behind last time I went to the doctors with him the doctor said his head was flat on one side and rondo more tummy time otherwise we might have to do a something about it like maybe a helmet? But he hates tummy time I just don’t want him to get behind I want him to lift himself all the way up using his arms and roll over he is very strong and holds his head by himself and has excellent control over it