This is tough as you will have to try to step outside of familiar cultural values - those that you see as appropriate for children to live by. I hope i can help a little, but i am just offering my perspective from philisophical background, not from similar direct experience - yet.
I see how too much information could seem burdensome to a child, but he should be given something to go on so as to not seem like death is forbidden to discuss or bad to think about. try to keep it light - or ask him what he thinks. tell him that is a good question, and try to go on his own ideas about it rather than focusing on your ideas. little ones are incredibly smart! he might have a very creative and innocent explanation for it all on his own.
if not and he is desperate for information, then just try to keep it simple, reference his familiar experiences with loss of loved animals and the idea that she was ill in her old age and is no longer feeling sick because she has passed and is no longer living in her sick body. "Her time on earth has passed, that is why we call it passing away... or she had a long fulfilling life and we are celebrating the times we had with her." If he is not familiar with your religious ideals, then it shouldnt be hard for him to understand in non-religious terms. Try to avoid the 'where did she go' follow up.
so if he does ask 'where did she go?', in non-religious terms death makes sense when we accept that its OK to not know what happens to someone after they pass - we focus on our lives on earth and we celebrate loved ones when they die. some people might think that is to open ended for kids, but in this instance it should just matter to your boyfriend - ask him if he would be OK with you talking it about it as an unkown. from an agnostic viewpoint, it is not possible to really KNOW anyway, so this perspective makes sense to non-religious people - where not knowing what happens to someone after death is OK.
if it gets complicated, you can explain that different people beleive different things happen to people after they die. This might get messy if you dont talk to your boyfriend first though - if you tell his son that you beleive in heaven, then you would have to also say that others dont beleive in it (and that is OK) so that he doesnt expect his dad to beleive in heaven. hopefully it will stay simple so to not have to go that far into it.
if it gets too complicated, tell him that you arent sure the answer to his question, and ask him to wait and ask his father the next day! good luck.