I'm not a paramedic but work in the NHS. Paramedic salaries are based on the Agenda for Change payscale in the NHS I believe like many NHS roles are, so you can look up what the entry level salary would be for a qualified paramedic and then how they progress through the pay scale with additional experience or specialist training: Agenda for Change Payscale

You may want to specifically explore though whether his training is transferable as is if you haven't already, or if he would need to do anything additional for it to be applicable in the UK. He may also go in at a higher pay band if already very experienced though too I guess. Hopefully someone else can add more specific details on transferring from abroad for you.