How do I get my Old English Sheepdog to be more active?

I recently adopted a (at least) 5 year old Old English Sheepdog January 1st. He has been checked by my vet and has a clean bill of health. I feel he is a little gaunt (skinny) and want him to fill out at least a little more. I don't want a fat dog in the least! I just want him at a good weight and in good physical shape. Right now he is shaved and weighs 76 lbs. He is a tall-framed Old English Sheepdog. I supplement his diet with glucosamine and chondroitin and Vitamin C, as suggested by my vet.

My perception of an Old English Sheepdog is that of being active, even until they reach 10 years old. I have seen older Sheepdogs romp and play and play tug rope. They also have a happy disposition.

My adopted dog seems to lay around all day. I take him for walks after I get home from work and to the dog park as often as I can. He interacts with other dogs fine. His fastest speed is that of a slow gallop to fast trot. He doesn't seem to want to exert himself too much in my opinion. He also seems bored at home, unless I give him a beef bone to chew on....

I realize he is not a puppy any more.....am I expecting too much from him?

Am I the one who has a perception problem? Is this NORMAL for this dog at this age to just want to lay around? Otherwise, he is a gentle dog who walks well and is house broken. He follows basic commands well.

How do I get this dog to be more active while inside the house? Is it really necessary for him to be more active when he is inside? Do I need to add anything to his diet to make him more active? Do I need to change my relationship with him for him to be more active? What should my tall-framed fullly grown Old English Sheepdog weigh? Does he need to gain MORE weight? I plan to have him grow out his full coat in the months ahead......

I know........I ask LOTS of questions! Try to answer most of them, if you can.....

Any help would be appreciated!