Correct grammar?

The second one is correct.

Les is a singular, so even though there is an s at the end of his name, you need to add an apostrophe and another s to denote a singular possession.

The second one is correct. Les is singular, therefore Les's name gets an s when make into a possessive. Plurals don't get the extra s. The only exception I ever learned was the Jesus does not get an extra s when used possessively. His disciples' names would get the extra s, though...

I would love to come to Les' house tomorrow

because if there's an "S" and you need to and an apostrophe you need to put it only and not add an "S"

Both are fine, but the as the reader, I can follow the first sentence a little bit easier. I prefer to not place the s after the apostrophe. Decide on which you like better and stick with it.

actually... the first is WRITTEN correctly according to grammar rules, but you are well within the your grammatic rights to SAY the second one (and I would encourage you to excersize that right, why would you want to go to "less" house?)

I would use the first one because that's how I learned to do it... but both are acceptable :)

Both are grammatically correct.

both are as the other person said 'gramatically' correct.

But, if I had to use one of those sentences, i would use the second one.

I would probably use the second one.