When does a sunburn need to be seen by the doctor?

yes

any compromise to skin intergity is an open invitation to infection and sepsis

at least call the doctor for advice

Blisters = second degree burns, which have to be kept covered with dry sterile dressings. The dressings need to be changed 3 times a day, and any kind of pus or foul smelling drainage is no good.

The more extensive a burn, the worse it is. But this sounds like a small area. Just keep it clean. It will itch something fierce when it is healing.

I think you should see your doctor sooner. A burn by itself affects the skin in different ways, depending on the patient. Here we have a sunburn situation and that sounds like your son most probably is very white and is very sensitive to sun rays. From your description of his feet, u have nothing to lose, prevention is always better than cure. Go check your son's feet at your doctor's without delay. And most likely, he'll give you another treatment which would match your boy's exact condition. Good Luck.

Better safe then sorry you do not wont him to end up with an infections, remember too use a sun Tan lotion in the future, hope he feel better.

I think you should go with your intuition. Why don't you call first and ask to speak to their nurse. She can tell you whether it sounds really serious.

I think you should, he could have 3rd degree burns.