Give him or her a bath. I advise talking to a vet. They have special shampoos for that and perhaps your dog smelling bad, could or may be a medical disease. but talk to a vet. they would know for sure.
I have 1 year old male Frenchie. He didn't smell really bad but felt sticky. Other than medicated shampoo I have a deep tub with a top on it that I add chlorhexidine (directions on jug that I get from vet supply) to the water and this summer I would dunk him every day when he went out. I have a floored run so he doesn't get dirty and it really made a differance with him. Don't know about any other dog. I asked my vet and she said she didn't see any harm in doing that.
I have 2 Frenchies and one seems to have that problem more than the other. I have to give my male frenchie a bath AT LEAST once a week. That usually resolves all issues in regards to smells.
It could be the food. We have 3 Frenchie's, and an American Bulldog, each with their own reactions to foods. We had been feeding them ground turkey (cooked) & mashed potatoes or rice. This really helped the gas and skin problems (due to allergies from food like yeast infections that makes the skin scaly and oily, excessive wax build up in the ears or dry, itchy build up in the ears.) About 3 months ago we started feeding them Royal Canin dog food & they've all been doing well. We are limited to the different types of specialty dog food where we live but have tried every brand, both over the counter & prescription, available. We went thru every type of protein & starches, raw food, cooked food - you name it, we tried it, to determine the allergens affecting our "babies". Our vet said to use Dial soap because it's anti-bacterial. We also use a tea tree oil based soap too- that works well too. We found that treating the allergies calmed the skin & gas problems in combination with the right topical cleansers (no need for all the expensive stuff - been there) has helped our Frenchie's to be happier & healthier.
If its a fishy, death type smell its the anal glands. Also, he might be farting more than you think, they fart a lot! Tell youre vet when he gets his next shots.