Looking for advice

Please be kind and no hateful comments I’m really struggling. About two years ago I had taken in a dog. The previous owner had not been truthful with me on this dogs history. I bring him home and long story short he has human aggression and is very fear reactive. He is completely fine with me and my family and some friends or strangers. However, sometimes randomly he will bite if someone tries to pet him. I always ask everyone to not pet, I keep him at safe distances and try to never set him up for failure. I’ve talked to some trainers and have tried to work with positive reinforcement. So now I come here with the question; what should I do that I simply cannot afford the type of training he probably needs? Do I just keep him crated when guests are over? I love him so much he is my best friend but I’m really at a loss right now. Any constructive advice would be greatly appreciated. Thank you