Get a belt. Stick it in your belt.
The ones with the D-ring shackles work well. Buy it a few sizes bigger to account for your padded jacket. You can always tuck in the end if it's too long.
https://www.amazon.com/Canvas-Double-D-Ring-Buckle-Metal/dp/B00EVB8AHY
If he has to dress up, just a simple leather belt that matches his shoes is fine. For jeans, just him a D-Ring canvas belt. They're tough as nails and go well with jeans or khaki shorts, and best of all they're super cheap if he loses it. I have some of these that I've worn for the better part of 10 years. https://www.amazon.com/Canvas-Double-D-Ring-Buckle-Metal/dp/B00EVB8AHY
Oh, I could’ve worded that better. Not getting hit with the metal ring, that’s just the type of belt I’m referring to (unless I’m thinking of something else). This was what I had in mind, sorry for the confusion. Which, of course it looks flimsy and not like the one I’ve got, but doubled over...?
It's a bit late, but if you haven't found a belt already, a D-Ring belt is the type you'll probably be looking for. I used one for my Okabe cosplay about 4-5 years back and it worked perfectly. I found one on amazon that's the style you'd be wanting to use
https://www.amazon.com/Canvas-Double-D-Ring-Buckle-Metal/dp/B00EVB8AHY
yeah, that's honestly the best route to go. hmm, you know i've never heard of someone buying belts from a guitar supply store, seems interesting. i, myself just use these types of belts i find them far more comfortable than the conventional hoop belt.