Check with your son's school about the practice clothing. Neither of my boys' schools provide clothing.
Regardless of a formal or informal school (I have one at each), you need to break it down into four groups; school wear, after class wear, sports wear, and sleep wear. Although many will lump school wear and after class wear together, especially at an informal school, trust me they change into to even more casual clothes for studying. Sleep wear is optional.
My biggest recommendation is shorts, shorts, and more shorts. I don't know about your son, but my boys and their friends wear them most of the year. If it's above 40 degrees, they wear them.