Stress affects everyone differently. Some people get cold and distant, others get angry and combative, and still others break down into uncontrollable sobbing. If your boyfriend was in that second or third category, you wouldn't take that personally. So why take his distance personally?
You know, deep down, that the underlying stress isn't your fault. If the stress isn't your fault, then the effect of the stress isn't your fault either. Please try to remember that and not take it personally.
That said, you can definitely do some things for him to be supportive. Ask him if he'd like to talk about the stress, and if he doesn't want to, then just remind him that you're there for him if he changes his mind. Beyond that, if there is anything you can do for him to make his life easier, try to do it. If you live together, do a couple of the "chores" he normally does so he can decompress and de-stress a bit. If you don't live together, just head over to his place with some dinner. Keep him company, but allow him to be quiet if he needs to be.
Long-term, your boyfriend needs to dedicate his efforts to finding the causes of stress in his life and trying to work them out constructively. If he's got a stressful job or isn't managing his time well, he should try to work through that and discover a solution that works for him. As you know, stress isn't just hard on the person, it's hard on the people that care that person as well.