You should have destroyed his base with your first loyalist attack. He had 2 PDs, but you could have walked around them and destroyed another 5 mexes and 4 pgens of his, delaying his rush to OC.
Once he has OC, your only option is to hole up and use air to try and kill him. Land units (short of XPs) simply will not work. To buy yourself more time to amass a large air army, you should build your base closer together (this you need to do from the very beginning of the match), get factory shields and build a lot of PDs (3 or more can usually kill and acu... build them apart to avoid splash damage.)
If you position your factories so that your PDs are under 2 factory shields, he will have a hard time killing your PDs.
Once you have 25 FBs or 15 gunships, you can attempt a snipe. Anything less only gives him more vet (he probably had enough RP to go AA gun). More is better, but if you're pressed for time you can use less.
