Why is he floating on his side then swims off again and he is eating?

He may be floating on his side as a way to rest or conserve energy, which is common behavior for some animals, especially when they're feeling safe or relaxed. Swimming off again suggests he is alert and ready to engage in activity, possibly to forage for food. Eating while floating could indicate he’s multitasking, taking advantage of a safe moment to nourish himself while still being aware of his surroundings.