Limit his freedom inside to a smallish area where you can supervise him closely and where the litter box is close at hand. When you can't supervise, confine him to an xpen with the box or to something like the kitchen area with the box.
Increase his freedom as his potty habits improve.
Great job with the praise.
Take him to the box immediately after each meal, when he wakes in the morning and after each nap, after playtime or anything exciting. Tell him, "go potty" or something using keywords. Don't worry if he doesn't potty right away. Say, "You don't have to go right now? Okay."
Clean the accident spots with an enzyme cleaner (ie Nature's Miracle). The scent needs to be completely eliminated or he will continue to return to that spot. Dogs have better noses than we do.
Does he seem to like the litter box?
Last, but not least, he is young. Keep at it and he will get it.