It can cause neurological damage but I have yet to hear of a rat actually dying from eating chocolate. Yes you are right on the fact that is can be a bronchiodialator and is good to give a little piece to any rats suffering from respiratory distress, however I have never done this. I would talk to yourvet first and see what he thinks, it is never a bad thing to call and vet.

If he starts walking in circles etc. you need to get him in to see a doc.

His feeding doesn't have to change, that won't do anything but harm him as he needs his veggies/fruits as much as his dry foods. But I don't think ONE day will hurt him, just there is no need for you to cut that out.

As far as him "growing up" Bucks start showing their mature side around 6-8 months old. If he starts acting spycho it's just hormones and he should cool down in a few months.