Do you mean just a nickname for Martin? Because I don't think just not liking Marty is good enough reason not to use Martin. If you don't like Marty, then just don't call him Marty. I know lots of kids (boys especially) who only go by their formal names, and it doesn't seem to be an issue. Or are you looking for an alternative name entirely? How about Marshall for an alternative?
I think Martin doesn't necessarily need a nickname, but I also wouldn't be surprised if an aunt or a friend called him Marty without your OK. And Martin might opt for Marty when he's older. So if Marty really makes you cringe maybe you need to forget Martin. If you could handle a teenager who is Marty at school, I say go for it.
Merit might be an alternate choice.
Also, Martin James could go by MJ if you have family members who just need a nn!
I think Mars would be an adorable nn, that was my first thought! I think a Martin could just be Martin these days, though--a lot of Daniels are just Daniel, not Dan or Danny, and the same for a lot of other names (Patrick, Henry, Benjamin), so I think a Martin could reasonably go without Marty. :)
Wrong gender, I know, but I have to say, I adore Martina and Marta on a girl!