I have a male kitten who is 8 months old; he is a cross between an American Shorthair and a Bengal. He hasn't gotten fixed yet. He's been the sun of my life acting all cute and what not, and has sufficed me in any of my daily problems as he is always there to cuddle :) Usually, he's affectionate toward me cuddling me and licking me, quite active and literally ALWAYS hungry. Lately he's been totally different. He used to take only 2 minutes to clean out his plate and come looking for seconds, now he seems distracted or uninterested. He follows me around while I clean his litterbox and he stops eating watching my actions or he goes somewhere to sit. He comes back to eat it eventually but it takes him 20 minutes to eat now and sometimes he leaves a quarter of his food behind. He's been urinating and defeciating normally, so I really don't get it. Instead of being his outgoing self he's so inactive: I know cats are lazy and what not and he was perfectly like that before but now... he just lies down and watches me for a straight hour. He gets up and walks around and whatever for half and hour, then he sleeps. He usually doesn't awaken unless I walk by. His loss of appetite concerns me as this could indicate a digestive problem, right? I'm not as concerned about his behavior though since he's unpredictable and he's attaining his own habits. Is this whole thing something I should be concerned about? Or is it simply a result of high testosterone? Or is just normal behavior for a cat his age?

Well this is a rather perfect example of growing up cat. My two cats I got as rescues and they were not even weened yet. They were very similar to your and strangely all of the sudden they became less active. I would still check you kitten for fleas because little kittens are highly susceptible to becoming anemic from loss of blood. I would also perhaps buy a toy or a long feather and try to engage him in play by tickling him while he sleeps. Kittens are extremely curious but after a while they just naturally calm down. One of my cats is a black american short hair and the other is a tabby both litter-mates and , like I said earlier, they calmed down in a similar manner.
Also a kitten grows they can become choosy about what they eat I would perhaps go find some small samples of different foods and see what he likes best. If you feed him wet food, I would provide him some dry food(a small amount) that he would have access to all the time.

He's reached maturity. Mature cats sleep at least 16 hours a day. Does he sleep longer than that?