My son is 9 years old with Down syndrome and he used to completely freak at the dentist. We have since found a dentist who works extremely well with him and it wasn't until age 8 that we were able to get him to sit in a dentist chair and have his teeth cleaned. He just had his tooth pulled for the same reason as your son and he too, had his first cavity filled at the same time.
We told him he was going to the dentist and he said ok. He never asked why, we didn't feel we needed to go into more detail at the time. When the appointment came up, he went right back for his appointment. The only issue we ran into was he didn't like the sound of the drill, so Dad went and held his hand and keep reassuring him while the dentist worked. As the dentist worked, Dad explained that there were germs in his tooth and the dentist was getting rid of the germs and making him a new tooth. They were in and out and home in less than 30 minutes.
I believe you can be honest, on a need to know basis, by only giving out the necessary information. If he asks more questions, then answer them as they come up. And of course, you know your child best and will know the best way to address this. Listen to your instinct as his mom and you won't go wrong!