My son eats any cereals he likes, but mostly Cheerios, Shreddies, etc. He weighs his portion (we have digital scales) and counts the carb content, adjusting his insulin dose to match. Since it's early days for you, you're unlikely to have established his appropriate insulin/carb ratios yet, so my advice would be to eat a consistent amount of carbs each morning and keep a note of how his blood glucose levels respond (ie if he has 2 slices of toast and jam one morning, totalling about 40g carb, get him to have an amount of cereal the next morning which equates to 40g carb - easy to work out from the nutrition panels on the box, and probably about 55-60g weight for many common cereals).
Cereals do tend to cause a spike in the BG level before it drops back down to normal - there are various techniques to deal with this, although my advice would be to initially just try the foods and keep a record of food eaten, insulin given and BG 2-3 hours later. Then this can be reviewed with the DSN. If he does have high spikes on a consistent basis after eating cereal, one technique to try is taking insulin ahead of time, e.g. 20-30 minutes before eating.
You will probably see lots on this forum about opting for lower carb foods or low GI foods. While these can help achieve a flatter blood glucose profile, they can seem very restrictive to a teenager, and good control can still be achieved while eating the diet he has always eaten. I do feel quite strongly that young people with type 1 have enough to contend with without feeling that there are things they can no longer eat!
