My 8 yr old child has severe bad breath and we've tried everything we can think of - he's had it for years.?

Could be tonsils . If you tried good dental hygiene , gargling , tongue brushing , then the tonsils could harbor bacteria. See an otolaryngologist.
Halitosis. You may consider having a someone look at his tonsils. The tonsil have crypts in them in which food can be stuck and lead to the bad breath you describe. The other issue can be his diet. If you look in his mouth and see large tonsils, visit an ENT for evaluation and further advice.
ENT. Try taking your child to an ear nose and throat specialist. It could be a foreign body.