How skinny is too skinny?

That's really hard to say. Can you get a baby scale from a garage sale somewhere? Then you only have to take him into the vets once to find out what he should weigh and you can do the rest yourself.

Do you have a regular scale at home by any chance? My scale does 1 decimal so I can get a pretty accurate reading. I will usually weigh myself 2x (to make sure the scale is showing the same weight for me) and then I will pick my cats up and hold them in my arms and weigh myself again, then figure out the difference. This has been pretty accurate in my experience (compared to the vet's baby scale).

My baby Chacho was pretty skinny when we brought him home. It took a couple weeks before he got to what I thought was a normal weight. I pretty much go by sight, if he looks too skinny then he probably is.  They way I see it, you should be able to feel ribs when you pet their sides but they shouldn't be jutting out. You should see some defintion from where the ribs end and the hips start but that defintion shouldn't be massive, if there's no definition then chances are the cat's overweight. SO if you feel their bones too easily/no meat on the bones = too skinny.   If you can't feel any bones/no definition = too fat.      Its okay for senior cats to have a little extra weight and it is typical for kittens about 5months old to look a little skinny and lanky (to a point.) They're in their awkward teen stage and like a lot of human teens they look a little lean and lanky but the rule should still be the same, just expect them to still look skinny just because their proportions are a little strange. (lean and awkwardly long looking legs lol)