One of the main problems about we, as persons who tend to block, is that we are not just trying to solve the problem but we have demands. One of the demands is that no one get to know about our situation. We have to solve it without anyone knowing we have a stammer in the first place. In that escenario you are making the problem much more dificult to solve. It like you are trying to win a boxing fight but you have to do it with one hand instead of both. Now, related to your situation, you HAVE to tell him or at least, stop being afraid of getting "caught" while stammering. Not just with your boyfriend or girlfriend but with anyone. It WONT solve the problem but it will make it more easier. That does not mean that you have to publish on your IG account or anyting like that. Just dont try to hide it. Its easier said than done I know. It´s been a hard one on me too. But take baby steps and at least with the people you care and proyect youself, try to be honest. I think you know the answer you just need the courage. When the situation seems to dificult start with little steps. Take care