Slowly for his dry so he wont notice
he is an addict so needs to wean off gradually.
I would suggest james wellbeloved, pah brand, nutro or joe & jills. You can source sample packets of most of these.
Is he greedy? If so then 3 meals aday would break things up for him.
You need to be guided by his tray results in regards to wet food. Something like poultry felix in jelly would be worth a go, alot of high meat wet foods are rich so its a double edged sword between going grain free and getting the richness.
If you get pouches it will be fresher and you wont be tempted to give him more to use it up
Its also important not to cut his food down too dramatically at once, and to weigh him once a month.