I do know people who use Grindr for friends. However, it's hard to read those messages you say he wrote and not instantly think, "Well, looks like someone's been fooling around," even if there's no explicit proof.
If he didn't say, "I didn't delete it" when you teased him about deleting it and if he used past tense, that's dishonest and deceitful. However, I suppose he technically could have deleted it and then reinstalled it later (unless I have the timeline wrong). But then I'd wonder why he deleted it at all; he didn't want to talk to his friends anymore?
It sounds like you weren't aware of these people. I always try (key word: try) to think, "Stick to the facts, don't assume either way," but the fact that he didn't mention any of these meetings to youwhen I was in a relationship, I would tell my ex about when I saw friends, how it went, etc. as part of sharing my lifestrikes me as suspicious. You not knowing of any of these other people, while knowing his other friends, is the part that really surprises me and seems like a flag. When he talks to you, see if his words line up with what you read in those messages and then go from there. Yet, there are a lot of unanswered questions that probably shouldn't ever have to be asked, so I'd say skepticism is warranted.
Even if everything checks out and there are no obvious lies, I think it's makes complete sense if you feel that the trust has been irreparably broken. Good luck. I don't know what is best in this scenario, but I wish you that.
EDIT: Oh, but also, people don't say "remember me?" to friends... If they don't remember you, then you're not friends.