Hi Karen, welcome to the DIS. Well you're going to the right country to get man sized clothes
Trousers: Pretty much everything will be available up to 40 inch waists and probably 50% or the styles up to 44". .
Polo shirts: In regular shops almost everything up to XL , 50% will have XXL and a few places hold XXXL.
Places I tend to do most of my shopping (I also tend to struggle in this country at times)
Reebok ( they do up to XXL shorts and swim trunks ) It's also worth checking out the golf section as they often have Greg Norman Polos up to XXL and tailored shorts up to 44".
Liz Claibourne for chinos, polos and "smart" shorts
The Disney outlets at Belz as they often have some nice Disney polos in XXL
Polo ( Ralph Lauren) for Chinos and often some really nice sports jackets and dress trousers.
Take PLENTY of spending money as it could be one of the few chances he has to buy smart/trendy clothes for a while. I usually do ALL my shopping in the US. Oh for things like "regular" white shirts for work and underwear
Walmart is probably the best value.
For larger than that you may be lucky in regular stores, but there are a few "high and mighty" stlye places in the Belz area of I-Drive