I think you should just stop trying and accept that different people have different tastes. It's not the end of the world if one of your friends doesn't like One Piece. One Piece is my favourite anime and manga but I'm the only person in my friends group that likes it, and I'm OK with that.
I'm assuming you're friends otherwise you wouldn't be trying so hard to get him to watch One Piece.
You can't force somebody to like something. There is nothing in the world, no anime, film, TV series, book or game that is so great that everyone will like it and if they don't they just haven't seen enough of it, that thing just doesn't exist.
If anything you're making it worse by trying to force the issue, if you've ever had anyone attempt to force you to enjoy something that you don't you will know that that just makes you hate it even more.
You think he can't get a good impression of the series from just 5 episodes but how many do you think he needs to watch? 10? 20? 50? What if after all that he still doesn't like it. How many episodes does this guy need to watch before you'll accept that he doesn't like One Piece or are you convinced that the only reason he doesn't like One Piece is because he hasn't seen enough.
There are few things more annoying than someone responding to "I don't like that" with "Oh you just haven't seen enough of it" it comes across as extremely arrogant and selfish. You give the impression that you are so convinced that your opinions are right that it's impossible for someone to think differently
Yea your friend's being a bit of a dick by constantly calling something you clearly love trash but so are you by trying to force him into watching something he doesn't want to watch.
For what it's worth I am honestly sorry that I'm being little harsh but it needed saying. You can't force someone to enjoy something, but you can accept that it won't ruin your friendship just because you like different anime. But if you try and force the issue it won't work he'll end up hating One Piece even more and he'll start to resent you for it. Just try to agree to disagree.
If you're still determined to force your friend to like One Piece and bare in mind I am advising against this get him to watch Strong World It follows the basic structure and tone of most One Piece arcs so gives the quickest and easiest idea of what it's like as a while. It's what I recommend to people who on the fence about watching it, but like I said that's for people on the fence not people who just don't want to watch it.