Prey size / is my corn underweight?

Yesterday I handled my corn for the first time since my move now that he had settled in and was fed. I noticed that he looked kind of skinny, or skinnier than I remember him being, but I didn't have my scale on hand and couldn't weigh him to be sure. He was about 88 grams when I weighed him at 6-7 months. I figured weight loss could be normal if he was stressed from the move, but then I remembered another thing I noticed last week.

When I bought some hoppers at the pet store last week, I noticed that they were a bit smaller than the hoppers I'd gotten in Florida. I assumed the NY ones were the "right" ones, because the pet store here seemed more knowledgeable/reputable than the one in Miami, but now I'm wondering if they were smaller hoppers or not hoppers at all. They are about the size of the thickest part of his body (not bigger though).

Anyway, my question is: should I feed him two of the small ones? I have two left and though I'm worried about regurg, I also don't want to let them go to waste! F/t mice in NY are more expensive than I'm used to...Or would it just be better to go up to the next size? What's the risk with feeding 2 prey items in one feeding?

The risk is a regurge. However, it will completely depend on the weight of your snake versus the weight of two hoppers.

Right off the bat, personally I don't feel that your snake is underweight. 88 grams at 7/8 months sounds great, so to lose a significant amount of weight (even from not feeding AT ALL) would take some time. Probably several weeks to even notice a difference.
If you notice your snae is skinnier, my first thought is if it has enough water (is it hydrated)?
Otherwise, it may just be your perception of your snake.

Your best bet is to weigh her/him. Then you'll know for sure what's going on.
Once you've done that, you'll know if it's time to go up a size in food. If you are unable to find the next size up in mouse, you can absolutely feed two of a smaller size, as long as they are in the same range for weight of the next size mouse... if that makes sense.

Here's a link to a good feeding chart: http://medusa-corns.webs.com/feedingchart.htm

I should have mentioned, I have fed 2 smaller meals at once many a time. Especially if my snake is inbetween sizes, or if I have a lot of left overs of the smaller mice and I don't want to waste them.

Thanks! I've actually been very worried about his hydration. All he does is hide under his water bowl and he never comes out. I posted about this and others assured me he was drinking when I wasn't around, but the truth is I work from home and live in a tiny NYC apartment, and I don't sleep much, so I'm always around or working close by his tank. I NEVER see him come out from there, let alone drink water.

Is there anything I can do to get him to drink more?

He probably does drink at night, and when you don't see (like what others have said). How new is he to you?
It took my corn snake 2 or 3 months before she came out when I was around, and even still she is quite shy.

All you can really do is keep clean water available, as you have been. If there are other signs of dehydration (such as sunken eyes, loose or wrinkled skin not typical of a shed) then you can put him in a bath; called soaking. It's where you get a container with a lid that has air holes, and put just a little bit of water in the bottom (but there shouldn't be enough that he has to swim!) for about 20 minutes or so. People often do this if their snake has stuck shed as well.
Unless you notice any signs of dehydration though, I don't think it is necessary. You may just be putting your snake through more stress.

Since it sounds like you have recently moved, I personally would just leave him be to settle in for another while.

Best of luck!