Hi Louby,
I think you will get many replies on this one, and I am as keen as you to hear other suggestions, but going on personal experience only, here are our snack ideas..
On diagnosis we were told anything under 15g carbs ok without injecting Novorapid. This soon changed to anything under 10g carbs. I have spent many hours trawling the supermarket aisles and reading labels! do remember that all fruit contains carbs, eg. medium banana 20g, medium apple 16g ish. So, if he fancies something 'normal but naughty' then a small bag Walkers crisps or Wotsits is around 13g. Nature Valley peanut 15g or Nairns oatcakes are tasty. Protein only snacks are good if he is 'starving' such as a filling cheeses n ham omelette, or a plate or cold meats or cheeses. Little cooked snack sausages maybe. I think the official line would be to stop snacking between meals, but for a teenage lad that's tough!
Looking forward to hearing other suggestions!
All the best