Weight gain - I feel like a cow, but I don't look like a cow yet!

I'm a pretty skinny guy (6'0" and 145lbs) and I'm at the point where the only thing that will make me gain weight is weight lifting. I eat a ridiculous amount of food for my size, but I never gain weight.

-Breakfast is usually pretty light. Hot chocolate, 2 granola bars
-Brunch is maybe a can of fruit (oranges, peaches, pears)
-Lunch could be last night's leftovers or a can of beans. 2 pizza pops if there are any
-Supper is whatever my mom makes. Usually meat or pasta. Side dishes are vegetables or salad or rice. A store-bought pizza if she doesn't cook. I'll eat 66% of that.
-The time between supper and bedtime is supposedly the worst time to eat. Everything gets turned into fat. Not true for me. I eat 3 cups of cereal (Usually very grainy/fibrous/healthy) with about 3 cups of milk. To top that off I drink a meal replacement drink. This is why breakfast is light.

95% of my diet is healthy food; lot's of protein and fibre. It's rare for me to eat fast food. I hardly do any strenuous physical activity, which is probably why I don't weight too much. I guess working out would most likely be the thing to boost my weight and muscle mass along with it. I just don't feel motivated though. :/

Jam it back in, in the dark.