Carrying on a relationship with someone because you feel sorry for them is the wrong reason to be in a relationship. You should be with a partner because they make you happy, are supportive etc.
A few years back I went out with someone and it became obvious pretty soon that although he was a nice guy, fundamentally, he was needy. And I actually felt really sorry for him so I continued things for longer than I should have.
At the time I felt that if I dumped I would a) upset him a lot and b) be a target for a lot more texting, messaging etc. So I put it off but it got to the point where (and I feel bad admitting this) I was repulsed by his neediness and basically, by him.
So I stopped the relationship and he did contact me to the point of stalking (luckily your guy can't visit your house at strange hours of the night!). But eventually he stopped and I realised after I dumped him that all he really wanted from me was my attention and when I didn't give it, he got unpleasant. Looking back I got nothing from the relationship, he was the only one that benefited. But I felt SO much better after ending things. If you feel in anyway like I did then I feel bad for you....but you can cut that negativity from your life.
Anyway, the point of what I just related was that don't let your insecurities get in the way. If he is the sort to send you abuse because you don't reply to an email for like a day then he does not sound like a nice person. You won't ever get thanked by him for the kindness and attention you show these sorts of people. They take and then want more and if you don't give it they get nasty. And if someone cared about you they should not be like that.