I just don't think there's any evidence at all to suggest that staying dry at night is a factor of "training" vs. out-and-out physiological development. So I complete disagree that financial considerations or "I'm proud of you" should have any bearing on it. I'm not proud of my kid when he doesn't get strep throat or when he avoids getting a headache or when he grows 4 inches, am I? Nighttime dryness is entirely developmental - when the kid is asleep, he's not responsible for knowing that his bladder is full. Period. My son had nocturnal enuresis (nighttime bedwetting) until he was about 11 - an unbelievable number of kids have this. So putting something that's out of their control and implying that it is up to them to fix can be really problematic. The other 2 kids are the same age or older, and they aren't dry at night. So why have higher expectations of your son (especially when wet-at-night is a bigger problem for boys)?
If he were wasting the pull-ups through play, or by intentionally wetting during the day, that would be something else again. That would be intentionally wasteful and of course you shouldn't support that. Wearing them over again is fine. When he's ready, he will let you know. I think you really should find another way to save money. We all have budgets and things can be really tight, but I would buy clothes at the consignment/resale shop and do hand-me-downs or exchanges with friends. I wouldn't let the kids waste food and I wouldn't buy things like those snack-packs (I'd make my own) or juice boxes (use a stainless steel bottle they can re-use). And absolutely no money spent on bottled water!
But making nighttime dryness a battleground can actually cause way more problems down the line. When you think about it, washing sheets and pajamas uses up water, detergent, electricity and it also wears out the fabric - so it's not really less expensive than pull-ups. I honestly don't feel this is a place to draw a line in the sand.