No, that’s not okay, sunflower seeds can be hard for a young pigeon to digest due to the large shell. They can get stuck in his crop.
I’ve had to force feed pigeons who didn’t like me, these were adult feral pigeons who were injured and not eating on their own. What I would do is hold them against me with one hand, making sure to keep their wings flat against their body so they couldn’t wriggle out of my grip. You can also wrap the bird in a towel if it makes this easier. Then, I moved my hand forward so my arm was keeping the pigeon in place and used one hand to open the beak and the other to put a de-shelled pea in the back of the beak. The good thing about peas is that they hydrate as well as provide nutrition, unlike sunflower seeds which are dry.
Also, make sure to leave some seeds and water with the pigeon once you’re done, it makes sense that he wouldn’t try to eat with you around. That’s why you have to leave some with him so he can try it when he’s alone.