I'm sure you haven't done anything wrong - you're clearly trying to help him
And if he hasn't been giving his diabetes the attention he should, then he's lucky to have you helping him.
There is an online carb counting course here:
http://www.bdec-e-learning.com
Or he could ask to go on the DAFNE course which he would attend in person. There may be a wait for that.
If he has Type 1 he should be seeing a consultant. If he doesn't have one, you or he could ask his GP for a referral. At a hospital diabetes clinic, he would have access to a specialist team.
In the meantime, encourage him to test more! If he doesn't test, it's like he's driving with a blindfold on. Testing and carb counting are the keys to good diabetic control.
@Kaychris Just a note to say there is currently a fault on the forum. I've been making sure your posts get through (I'm a moderator here) but I'll be going offline soon, so if you try to post and it doesn't work or you get error messages, please don't worry. It's not you. It's happening to a number of people. Your messages will appear eventually as we get them through manually.