^but at 5 months... not a good idea, you'll hurt his bone growth or tear a muscle

long walks by the lake sound nice
or how about going for swim
when he gets older you can learn to bike ride with him- I find it tons of fun to do with my dog!

but if all you're looking to do is beef him up with weight- puppy food itself is high in calories; it will help, are you feeding him 'large breed' puppy food? b/c that makes a difference too