my son is 12 and still in diapers. He has a meltdown every time the toilet or changing his diaper comes up.
It would help to catch him consistently. When there are any clues he needs to go or is, (This worked for my younger moderately to severe autistic sister, potty trained at 7) you reward him/her with something after they use the bathroom. Saying "first go to the bathroom and then(what ever the reward would be)". To avoid the meltdowns. and bring him to the bathroom and provide them with what ever else assistance he needs. reassuring him that he will get his reward, so it keeps his mind of the meltdown. Also maybe schedule bathroom breaks with what ever consistency you find needed to further encourage him. Never scold them or punish them for having accidents. It does take time though (all depending on the child) to understand and get the routine down.
Hope this helps