skinny sleepy cat

Question
we have had JAsper for over a year now and he is SUPER friendly, and fat.  I switched his food to Acana brand.  Now about a month later, I realize he is SUPER skinny and not so "jump in your lap"  he just stays in a quiet place alone.  We have a busy house, so I'm worried that maybe this has been going on for longer than i should have noticed.  I think back and he was puking a few days ago.  I'm on a tight budget tho and would like advice if this is worth a vet visit, or is just something that will blow over.

Answer
Cats don't lose weight that easily or that fast unless there is a serious medical issue going on.
When cats lose weight too fast they can develop a problem called fatty liver which is very serious. I don't know what this food is you are feeding but it must not agree with him and it sounds like he is lacking in energy because he isn't getting any from his food.
But since we are talking about a cat, I am more worried about his liver.
For that reason alone, I would head over to the vets and have him looked over.