First of: Let him know that you need him to be around, not just physically, but mentally (meaning be there with you, not the computer).
Let him know of your feelings, even if you don't tell him of your desire to cut, he should at least know you are hurting inside. Let him know you need him to support you (and that that doesn't have to mean buying you lots of stuff, but simply being around and showing affection, if that is what you want from him).
there is also a chance that he has some idea that you are feeling low, but retreats to the computer because he thinks you want to be left alone. - it's hard to say
_ and try to find activeties that keep you from hurting yourselves if you feel the urge building, - like going for a walk or taking a cold shower or something of the sort.
