Don't do it while they're eating, and try to avoid it in open spaces with a lot of people. Especially good if it's just you and him in a quiet corner- because if he does think it's a prank, he'll look for onlookers.

Again this year. My aunt got a shirt that was just obviously mislabeled. Said 6X, more like a 2X. Ended up giving it to someone half my size that night.

Maybe it's just me, but the gaps between actual sizes shrink as they go up- like a 4X is closer to a 5X than a medium is to a large.
Also, for practically, find a good store or two and keep in mind vaguely how their stuff fits on you. Online is not always your friend- I ordered a too-cheap 6XL bowling shirt once, and their wacked-out standards would have put me in a 9 or 10XL.
P.S. in case you didn't already- avoid polyester in the summer as you get bigger. What becomes slightly warm will instead be very hot.

Hanes boxer briefs. If you've got fat thighs like I do, the legs give out after a few wears. Then, they prevent rubbing, while holding everything in place decently.

I wonder if it's somehow site visitors (like, not signed in) getting clocked in error. I get more no-profiles than regular users looking at me.

Eat lots, and lift heavy. If you don't have weights, try one end of your couch.
I've been fat my whole life, and I'll tell you- once you're fat, the leg muscles develop themselves. Just keep walking.
Eating lots of proteins, carbs, and sugars will get you bulked up.

Based off your profile height, what you do might work.
But, at 6'3" or whatever I am, I find that most shirts don't go down far enough for my gut. So if you get larger, you might find you need to put your belly in your pants.

If you continue getting bigger, you're going to want to consider the rise of your jeans. They'll need to be deeper in order to hold your gut in.
As for budget options, I don't know what all is possible in ON, but try Penney's clearance, or even thrift or consignment places.

I have a big gut, and no ass, but I don't ever have this problem... I use a thick, wide belt and it holds up my pants fine.
Maybe it's a question of belly fat density. My gut is pretty solid.

Not at all your height, size, or even gender, but I'll just say you can use mybodygallery.com.
Pick your basic body shape and height, then increase the weight.