Hi mum! From what I've heard from other parents, small movable lumps behind the ear are usually just lymph nodes, they're super common in babies and totally normal, especially at this age when their little immune systems are working hard. Most mums I know who've had the same worry got reassured at their check-up that as long as it's soft, pea-sized, and moves around freely, there's nothing to worry about. That said, good on you for getting your son checked with the doctor anyway, they'll be able to measure it and make sure everything's fine, and that peace of mind is always worth it!