An intelligent dog needs positive reinforcement training, dumb dogs need to be disciplined.
When he gets down from the chair as instructed give him lots of happy praise and a treat. after doing this a few times he'll catch on. as for the chewing he's a puppy and he'll chew so the things you don't want chewed on keep up, when you find him chewing on something you don't want him chewing on tell him NO! and give him something to chew that you approve of.
And he will learn he gets approval and a treat for getting out of the chair.
Get him a soft doggy bed and put it by the chair. Get a rolled up newspaper and swat him out of the chair. Much noise, no pain.
Thats usually not what they learn with positive reinforcement, they seem to learn the last action they did when they got the praise which would be getting out of the chair when commanded.
I've trained dogs both ways and am surprised at how well my newest dog reacts to it. Praise from me is the thing she seeks most in her life now and is willing to do anything to get it. I taught this dog to "Shake" inside of 5 minutes and she hates her feet being touched. She might even be a little smarter than me..........
I also have a dumb dog, he's about as smart as a bag of hammers, he loves praise but is willing to ditch it to do what he wants. A good swat puts him back in line for awhile. The only difference I see between the two in the training aspect is the intelligence factor.
