8 months, depending on his mental age shouldn't be too long, or too serious. Better to do it now, rather than later(because that really sucks).

I don't know if you should say "I'm not sexually attracted to you anymore", because that WILL hurt a man's ego. Say something along the lines of; I don't want to get serious or I don't think our relationship is going to go much further, so I should just cut the cord now.