It would be useful, wouldn't it, to have ready a sound-bite answer when somebody asks something like 'What is a Christian?' - or, more likely, 'OK, then what do You mean by 'Christian'?'
There is an answer, in two short statements - which is stated clearly and repeatedly in the Fourth Gospel - and that answer is something-like:
1. Jesus is the divine Son of God who brought us the possibility of eternal resurrected life in Heaven. 2. If you want that; then you should believe, follow and trust (i.e. love) Jesus - though death into eternal life.
That's all that needs to be said. But a modern person will almost certainly want to know 'But how can I know this is really True?'
He will suppose that this means he needs to hear the 'evidence' for these claims...
But whatever kind of evidence you provide, it will Never Be Enough.
Because modern people will believe anything nonsensical or false that is mainstream or fashionable (e.g that people can change their sex); and can disbelieve anything anything true and vital on the basis of whim, convenience, expediency - or for no reason at all.
So - forget about 'evidence'.
Rather than getting enmeshed in trying to convince him with history, scripture, church authority, philosophy and logic, science or anything like that...
The proper and effective answer is that the only thing which could potentially convince would be when he discovered it for himself, by thinking seriously on the matter.
Nobody else can do it. If the thinking is not serious, it won't work. If he does not want to do this, or cannot make the effort - nothing will happen.
You can tell him what Christianity is. You cannot make him a Christian.
If he does not want to be a Christian - he won't be.
If he does want to accept Jesus's gift, but is unsure whether it is true - then nobody else can (or should) convince him of the fact.
If he wants to know - he can.
But it does require effort.