My son has a swollen lower eyelid I thought it was a stye but when I pull the lid down it has a sort of large white head on the inside

What you are describing is a stye. It will break on its own, but if you are not comfortable with it, see your doctor, who will probably send him to a specialist to have it lanced.

Tick

Apply warm, moist compresses (a warm, wrung-out washcloth) several times a day for ummmm 15-20 minutes each time. Maybe read him a story or sit and chat with him during the compress time.