6-year old cat is teething?

My cat is 6 years old and acts like a 2-month old! When he was younger, all he did was lie around and be lazy and cute. Now, he acts like what I believe he should have then. I know he's too old to be teething, I just couldn't think of another word because he's not really chewing things, just... show more My cat is 6 years old and acts like a 2-month old! When he was younger, all he did was lie around and be lazy and cute. Now, he acts like what I believe he should have then. I know he's too old to be teething, I just couldn't think of another word because he's not really chewing things, just nipping/biting them and then rubs against it. Also, he used to be lazy and fat(not overweight just chubby), and he's started to lose weight and has endless energy. Here's a list of things he's been doing recently:
-- He'll walk up to me, get on his hind legs and put his front paws on my leg and meow
-- No matter what I'm doing, he'll bite (Gently, like a nip, but sometimes it does get harder) my knuckles, fingers, knees, elbows, nose, anything that pokes out.
-- When I walk, he'll chase after my legs and try to get me to stop
-- He won't eat out of his bowl and chooses to go into the pantry and shove his head into the bag and eat from there.
-- When he can't get into the pantry, he eats my dog's food (Beneful) I know he likes his food though and my other cat, I have assumed is his sister, eats it too.

I think that's it. I'm not extremely worried, I just want to know if I should take him to the vet. I love my baby boy and don't want him to be in any pain, like if his teeth hurt or he has joint pain or something.
Update: He is neutered and loves his sister very much! He isn't unfriendly toward my dog, but he isn't friends with her either. They usually just don't go near each other.
3 answers 3