It means that someone is in an extremely altered state of mind after taking drugs. People may take so called "recreational" drugs to "get high" meaning some kind of pleasant feeling or sensation. "As hell" is used to emphasise something, usually with a negative connotation. For example someone with a hangover might say "I feel as rough as hell", meaning that they feel extremely unwell. Although it can be used in a positive context as well, for example "He is as smart as hell" meaning "He's extremely smart".

Without more context, it's hard to say. "High" can refer to altitude or it can refer to being affected by the use of a drug such as marijuana. When you say something is "(as) _____ as hell" it means that thing is very ________. For example, if I warn you that my neighbor's dog is (as) mean as hell", it means the dog is very nasty and mean.

I have a feeling that it implies that the man is on a high: essentially he has taken a drug of some sort, possibly, and is now exhibiting some strange behaviour. Could you provide a bit of context?