To a degree he is being stopped. The sanctions that have come in will wipe out the country and probably ensure the programme is kerbed altogether, the people of NK are already starving so how are they going to afford a nuclear bomb after this? NK has only tested 1 bomb that seemingly has the capability of doing considerable damage and they have yet to demonstrate how they will fire it from NK to somewhere else. The launches they are currently doing are essentially hollow cans with rockets strapped to the bottom, they have yet to integrate their warheads onto a missile let alone demonstrate it can even hit a specific target.
You have to remember that much of their technology will be derived from the Chinese and the Chinese are showing an increased willingness to halt their dealings with them. It takes considerable resources and effort to build a nuclear weapon and where is that now going to come from given they are sat at the top of every single banned arms export list? They will receive no help and they will soon have no money to buy that help.