Aw I feel for you, I really do.
I was you last year, also a 18 month relationship. There was so much which was so perfect while we were physically in each other's company. Like you helping with support through troubled time with teen without getting too involved or imposing. We enjoyed eachothers company and always done lovely things together. Seemed to want the same out of life and were planning to buy a house together.
But then also like you, I noticed he had lied about his whereabouts 1 time. I pointed out he couldn't have just left supermarket where he said he was as it was a Sunday and it had shut an hour ago. Then he showed a screenshot of Google maps to say he parked there but had gone for a walk round and round a park. Didn't understand why he wouldn't just say he was at that park instead so it seemed suspicious, but still to this day don't know if he was with anyone, so moved on from it. Then more and more lies about his whereabouts come. It doesn't stop after it starts and they get away with it once. Mine was actually the hardest breakup I have had because the good times were still so plentiful. Our very last day together was fantastic and I felt so happy, but then the next day found out about more deception. He was like 2 different people, the guy while he was around me. Then this other guy when we had days apart. I couldn't date that apart guy but couldn't have the together guy without the shit guy in tow so both persona had to leave for my sanity.
I have no idea if he ever physically cheated. He said he had never cheated on anyone, but i do know that soon as we split he was with someone else that very next week, so he at the very least had an emotional connection with someone while we were together to have this backup in place.
My advice would be to end it now, to save you from the months of second guessing your thoughts and sanity when you can see things are not adding up. This is just the thing you have found out, how many other times has he lied about where he been which you will never know about.
I know it is horrible and hard when you have so many things that are right. But with a wrong 1 you can't just have the good bits. You also know he got form for cheating. So that will only add to the risk, doubts and trust issues here.
Believe me I know, now you have found out this lie, there is no going back. You can stay and wreck your head for the next few months, or you can get this over with now.