FLR,
He would go over to the Captains roster on his natural seniority, e.g. PP23 after 5 years frozen, NOT at PP18, the seniority moves up regardless it just means that the annual incremental payments would be frozen.
I am not sure what would happen regards to back pay, if he didn't bid thus leading to the freeze then I believe he doesn't receive any potential back pay from missed increments.
If he bid for commands he would not have been frozen but if a mistake had been made and junior pilots had been given commands before him then the difference between when his command was achievable and when he was awarded his command course would be back paid.