wheezy breathing

Archie does this weird breathing thing and I have no idea whats wrong with him. He rapidly and niosily breathes in and out- like he's wheezing, and his body shakes, but it only lasts 5 seconds and the he's fine.
I wondered if he was coughing up a hairball, but he's been like this a few times and nothing so far.
We took him to the vet who said that some cats are just like that, and some pedigree cats are prone to it. But he isn't pedigree...
Also she said he was underweight- he's about 51/2 months and weighs a little over 7lb, what should he weigh?
Help! Whats wrong with him?!

Can you tell where the wheeze is coming from? Nasal passages, lungs, throat? Asthma is always a worry- I'd try and get film of him doing it to show your vet. I'm a bit surprised they didn't take it more seriously.

And some cats stay small- don't always focus on weight, feel his ribs. Are they boney, or covered with meat but you can still feel them?
If he is too thin, rule out intestinal parasites then just offer him a good wet food often and maybe free feed a high quality dry food.

Some cats just tend towards thin.

I'd go with throat- it doesnt sound chesty. The vet said he probably just had a throat infection and gave him a jab but he's still making that noise.
Him and Tilly are sitting on my lap atm (my lap is too small for the pair of them but somehow they manage )
I can feel the bumps of his ribs and spine but thats it. He's a sleek cat so its easier to feel through his coat than another cat's, but he does have a nice layer of muscle/fat in between my hand and his ribs. I think he and Tilly are just little cats- Tilly is Tiny! He has some Aby and Bengal in him but I didn't think they were especially small cats.
I'm still worried about him, and I don't like the vet he saw very much but I like the other vet who works there too much to change, so I'll try and just see her from now on.
They are indoor only cats who have both been wormed.

He probably has asthma; his symptoms are very similar to Gizmo's.

You'll have to get a HEPA vacuum, vacuum the floors every day or at least twice a week; and it really helps if you get a room air filter. Gizmo's troubles have nearly disappeared since I got two of them. Good luck

I agree - it sounds like asthma - my Wellington has attacks like that, less now we are in France and the air is purer. I control it with prednizone, prescribed by a vet. He would need an Xray to confirm, but it can be very dangerous for cats so if it continues I would insist on further tests.

Thanks for all the advice, if he does it again we're taking him to the vet.
I'll keep you posted