I honestly cannot understand why not go for the orthotics, Its not like he will have an operation, or something that is totally irreversibe.
Podiatry has come a long way in 20 years, and if I had been offered at 3 years old to get orthotics, for less than $100 to save my feet then I would have been saved literally thousands of dollars as an adult. You are lucky he is a boy and not a girl because boys shoes are broad and comfortable.
I have about 3 pairs of shoes because I hate buying shoes because nothing fits and they are all uncomfortable. I have had orthotics for 20 years, and without them I would have back and knee problems My daughter also wears orthotics, and they help with foot pain as she pronates too. The only shoes I ever get for my kids (and myself) now are New Balance because they have so much more technology in kid's shoes than any other manufacturer.
Sure orthotics are shoe size specific and you will have to get new ones when he outgrows them. But you have to buy him new shoes too when he outgrows them, right, and clothes, its a normal part of being a child. My advice is don't skimp now because he will be the one regretting it later.
Regards, P.