It's okay for at night... even older kids still have accidents at night.. sometimes they are just sleeping so solidly that they don't realize. Once my daughter, who is 5 yrs., pee'd in the bed... and she didn't realize... she said she was having a "dream" that she was pee'ing but didn't realize it was "real" and then when she felt wet, THEN it woke her up. LOL.
I still use waterproof bed pads under my girl, just in case. It's good to get several as back-ups and just change them out, launder, etc.
MANY kids this age and older, still use some sort of night-time diaper...it's not unusual. Sometimes they are just so tired, they can't wake up. Think about us adults.... we have a hard time waking up in the middle of the night too... whether for them or for us to go to the bathroom. LOL.
Accidents are going to happen. There will be these moments. If it happens with my girl, we just clean it up, she puts on new pajamas, I put a new bed pad under her, we don't scold. It's just an accident.... versus a "step backwards."
I think your son is fine.