How would I properly socialize my dog?

So I was wondering if there was any way to socialize my dog even though he's five years old. I got him in the beginning of my freshmen year in high school and i took him to puppy cles and he knows a slew of tricks. But due to my immaturity I didn't keep up with his socialization. It wasn't too bad until he and I were on a hiking trail and he appraoched another dog and got bit. It wasn't bad, the dog didn't break skin but there was a lot of saliva on my dog's neck. It must have affected him because the next time we went to a vet a dog approached him and he lunged for the other dogs neck. Since then I have been a little wary of other dogs meeting him. I want to go back to hiking but I'm so scared there will be an incident where my dog is at fault. Wondering if anyone had tips or knew how to resocialize a dog. Thank you and sorry for the long explanation.