We did the same thing, and the portraint came out WONDERFUL!
Cruiselineformal.com is the official tux rental for
DCL. Any others you'd have to make arrangements ot pick up and return off the ship. I thought CLF was fairly reasonable in cost.
For DS you'll use whatever size he wears in regular clothes, unless he's out of boys sizes or husky. So if he's a size 16 in his regular clothes, that's what he'll be for the tux.
For DH you'll need to take him to a local tux place to be measured. Generally it's either free or $5. Now, this was my experience with THAT...
The measurements they came up with I called in to CLF. The girl who took the order thought they were very weird, and asked me to double check. Luckily DH had recently bought a couple of new suits which still had all the tags on them, so I was able to to look at the sizes. The tux place had screwed up the measurements ROYALLY. The girl from CLF was wonderful and suggested using his jeans to figure out the proper inseam length after realizing how badly the tux shop had messed up. (They had him in a jacket for a guy who weighs 300+ lbs. and pants for a guy who is about 5'2"--that's is certainly not quite right for my 6' tall marathon runner DH!

)
The point of the story is that CLF was WONDERFUL and caught the discrepency in the measurements, which not everyplace would have done. They were also very personable, and I'll use them again without question
Anne