Jotun Grunt + Goblin Welder Rules question

[Dominik]

OK, so here's the situation:

I'm playing UW fish against Stax, I have some creatures, lands, etc out, as well as a Jotun Grunt with 0 counters. He has 3 welders, a Smokestack and mox emerald in play, a crucible, wasteland, and granite shard in his GY, as well as some lands. He passes, so it's my upkeep. Here's where we get into a debate: The grunt ability is on the stack. He says that I have to name the cards that have to be put back on the bottom of his library right away, and then he could switch them with welder to make the grunt die, while I say that I only name the cards to be put back upon resolution of the cumulative upkeep trigger. Who (if anyone) is right?

Thanks for the help!

[Dante]

Umm...the search function is "tech" or just looking at the first page, you'll see this question was answered like 4 threads from the top here.  Verbal Warning