Lots of dogs don't bother trying at first, especially pups who are used to their food arriving at regular intervals, conveniently laid out in an easy to access bowl. 
To begin with put some particularly tasty/favourite, treats in there but only loosely. Help him roll the kong around a bit and show him the treats fall out as it moves.
Next time do the same, then smear a little of something super tasty just around and inside the large hole and hold it while you encourage him to lick it off.
Next step is to loosely fill it with something extra tasty, perhaps some fresh cooked chicken or sardines/pilchards and again encourage him to eat it.
Eventually he'll get the idea that he has to get stuck in to get tasty treats of out it.
My older Lad was totally disinterested in kongs this time last year, but I went through the above stages with him and now he'll happily demolish a frozen kong (full of sardines in tomato sauce, mixed with kibble and topped with some Lily's kitchen wet dog food) in under 20 minutes.
Fwiw, kong paste isn't all that appetising to a lot of dogs and isn't particularly good for them either. Far better to use something healthy like tinned fish in tomato sauce, some fresh cooked meat or one of the better quality dog foods like Lily's Kitchen or Fish4Dogs Salmon Mousse.