You cant see his keel bone but I will feel it to see if its sharp.

You should always be able to feel the keelbone - if you can't it means the bird is too fat. But it shouldn't stick out too much either. I don't have any talent for judging it myself.

I can feel it easily I dont know what it means by being sharp because real sharpness would probably break his skin lol but yeah I can feel it and by just looking at his stomach I cant see it but I see a line through his feathers.He is having his dinner now Carrots,Watermelon,Sprouts (the real sprouts Brussel sprouts lol.) a little bit of potato and lentils and the broccoli grassy sprout things.I saw a nice big weighing scale yesterday in the euro shop,cheap enough so ill get that.

A scale can tell you what your bird's weight is but it can't tell you how appropriate that weight is for this particular bird. A scale is very useful to have though. If you check your birds' weight regularly (like once a week) you'll be able to see whether there are any alarming trends, like a major weight gain or loss. If you breed your cockatiels next year, it's best to weigh the babies daily to make sure they're growing and gaining weight at an appropriate rate.

Ill probably be weighing all my birds every day lol I find stuff like that fun lol.Ill check what the average weight of a plum head is and if hes lower than that ill know something is wrong.

It doesn't necessarily mean he's unwell just because he's smaller for his age. But yeah, it is a good idea to weigh birds regularly.

I dont really think hes small I think its just his underweight thats making him look small.