Honey, if you don't like him you don't like him end of story. My friend has just recently been in a situation where she was messing about with someone who had a girlfriend, and cos he said he was never going to leave his girlfriend she started seeing another lad 'as a distraction'. This lad was really lovely, a really nice gentlemen, never tried it on, picked her up, took her everywhere. But at the end of the day she didn't fancy him, and she's just had to call it off because he was getting attached.
It's cruel to go out with someone you don't want because they'll always feel more for you than you do for them. Mum's always want there 'little girls' (cos that's how she'll always see you!) to go out with 'nice' boys, so you can't go out with him just because of you mum saying so.
I think the best thing you can do is be honest as you can with him without hurting his feelings. You could use the fact that he's joining the navy and you wouldnt' get to see him that much, so it wouldn't be fair on either of you. But you'd really like to stay in touch as friends cos you think he's a nice bloke.
Lads don't like it when their ego's get dented, that's why the other two fell out with you for a while, but if you let this lad think that it's all for the best he should be fine.
Hope everything works out okay. Make sure you let us know.