I have a rednose pitbull who is three years old and he is not gaining weight. He is skinny and his ribs show and I really want him to gain weight and not look like that I feed him constantly but it's not changing. What can I do to help him gain weight? What do I need to help him gain weight? Please help

Firsly, you should have him checked by the vet to see if it's nothing medical.
And in what way do you want him to gain weight? Fat, or muscle?
And gaining weight will depend on what food you're giving him. A high protein diet would work the best.
They have vetenary experts on this website who would be able to help more as soon as they come online.

And also, you could check out previous threads that people asked about dog needing to gain weight. Should be just below your question and the given answers.
Under the titel "similar questions"