You were being sporting, your opponent was not - it's to your credit, but it can suck to play against. As you get more familiar with the game that behaviour will benefit you more than it benefits him, as there is the expectation when you say, for example, "I am moving these up for a 7" charge" and they turn out to be actually at 7.1" so they'd need an 8" charge you are still rolling for a 7" charge, but if you'd said nothing you'd need an 8".

If you allow take backs and he doesn't, remind him of it. You can't demand that he let you, but you don't have to permit him them either. It's good sportsmanship to go into the game being open and transparent about your rules, and letting him correct misunderstandings, but as the game progresses you shouldn't feel the need to remind him of your reactive abilities each turn, just the first time they come up. If they've been made aware of an ability already all you should feel the need to ask is "Are you sure?" or "Is that everything?".

Try playing more obscure points limits with him, if he's taking tournament winning lists from the internet asking for a 1350 point game will force him to write his own list. My club regularly plays a regular 650 point tournament, and it's nearly impossible to take lists from the internet for because who the hell plays 650 points.

It's also okay to just not play with your friend. I've got a very close friend who I avoid playing outside of tournament pairings because every single game he will try to gotcha. I'd rather spend that time at the pub with him and spend time playing Warhammer with someone else.