You might start by taking the pads outside with you when he goes outside, then either stick it under him when he goes or wipe some of his urine on the pad, then bring it back inside.
You could do a reverse training of sorts, like some do when transitioning from pads to outside.
Slowly moving the pad closer to inside. I would start over as if he is a baby and praise praise praise whenever he pees on the pad, outside or inside. Or, in the beginning, even if he only sniffs the pad... praise and treat
and repeat.
Using a command, such as "go potty" and taking him to the pad after eating, drinking, playing, etc. You should be familiar with about how long before he has to go again, and take him to the pad and put him on it and give the command
It probably isn't going to happen overnight, but if you keep with it, persistent and patient, he will learn
Are you letting him go outside when he has to go right now? Does he give you a signal, go to the door, etc? If so, you can use that as a cue for his training