You could start the week with his old goggles and see how he goes, getting OTG ones in resort if the regular ones steam up lots. IMHO a helmet makes the steam issue worse as there is even worse ventilation round the top of the goggles. Last year was the first year I wore a helmet... and I'm going to replace my OTGs this time after trying various combos of all the goggles the family own last year.
I wear glasses. I have used "Over The Glasses" goggles , with a single skin lens and also "regular" goggles with a double skin lens... I'm not sure there is much difference in comfort. The difference is in steaming up. The regular ones didn't have such good ventilation and my glasses steamed up more than with the OTG set.
I'm planning on buying another pair of OTG goggles in resort this winter ( foam has perished on old set) . They are cheaper and there is more choice in resort than in the UK.
My DS is 7 and has been wearing glasses for a year now, mildly short sighted. Last winter he skied without glasses and just wore googles as he was not used to wearing the glasses all the time anyway. This year, he will be skiing more on blue runs and is used to wearing glasses, his prescription has also increased slightly.
It has just occurred to me that I will need to sort out something for him. Has anyone got any recommendations for over the glasses googles or other alternatives? Really do not want to splash out on anything expensive.