Probably dumb phototherapy question

[Imapsomom]

I notice tonight the boys rear end and lower back (areas that have never had sun exposure) are quite red and verging on burnt. We are due to go in for the third treatment of the week tomorrow. I will ask at the dermatologist office but I would appreciate your thoughts on a) do we wait until Monday and only go twice this week or b) go and ask for a lower dose snd less time so he is still getting the 3x uv but not getting a burn. It seems obvious to me to skip tomorrow, but I don’t want to lose and benefits (and time spent)