I'm in a somewhat similar height/weight to you. 6'0 and 143 (last I checked 2 days ago). Something you could try is a modified GOMAD (Gallon of Milk a Day) diet. I know you're lactose intolerant, but get this, LACTAID®
is almost identical nutritionally to regular milk. If you can get past the unappealing name, then it may be a way for you to help put on some extra pounds. Info on the GOMAD diet
. More info on weight gain
you might be interested in. Actually that whole FAQ is awesome.
EDIT: Also, snack on peanuts/almonds/nuts in general throughout the day for extra calories + extra protein.