I would do 2 things:
First I would get a vet checkup JUST to make sure everything is ok. I suspect everything is just fine.
once that is done, suspect 2 other things:
1) at 7 months he may be slowing off a growth spurt, and he may not need as much food.
2) he may have trained himself to not have good canine eating habits by having too much food available for too long a time period.
One must know a little about how life for a wild canid works to understand why free feeding and/or leaving food with a dog too long, and ANY help you give ANY dog to eat which is not ill or injured, can be a bad idea.
Wild canids are both hunters and scavengers. They have evolved like most predators, to eat when food is available, and to eat whatEVER it is. When dogs are allowed too much food, or allowed to have food available at all times, they may "forget" their good canine instincts that tell them to EAT IT ALL and EAT IT FAST whenever food appears. This is also what makes dogs such inveterate beggars.
So it's possible that your puppy has had an appetite reduction, and not a corresponding reduction in his food.
Here is how to handle it:
Take his dinner to him tonight, put it down, and leave it for 3 minutes.
Use a timer. Do not stand over him, give him his privacy to eat.
When the 3 minutes are up, take the food up, measure what is left. He will get half this amount, however you will skip one meal. I assume you are feeding him twice per day. If he leaves ANY food, he skips the next meal, and gets half what he left on the following meal. When the dog starts cleaning up his bowl with gusto, keep an eye on his body condition, and adjust food as necessary.
Remember:
*No hand feeding.
*No begging him to eat.
*No helping him to eat.
*Remove food and bowl after 3 minutes UNLESS he is still ACTIVELY eating.
*SKIP the next meal if he leaves anything in his bowl
*Cut his food amounts in half until you get to an amount he will clean up briskly
Good luck, it's a hard issue, and terribly hard not to give in to those eyes that want that hand feeding, but it is so much better for him if you train him to eat like a dog again.