Taught my nephew to weld today, first two beads ever on the practice plate

That jump from the first bead to the second is real. I taught my daughter on some scrap angle iron last summer and her third pass looked nothing like her first. What helped her was me holding a scrap of plate next to her work with a line drawn on it so she had something to chase. Chasing a line got her travel speed steady way faster than me just saying 'move slower' over and over.

The watching the puddle thing took her a few sessions to figure out. I told her to pretend the arc is just a flashlight and the puddle is what she's actually looking at. Once that clicked her filler dabs got way more even. First time she tried rod she kept freezing up and her bead went wide and flat, so don't be surprised if the nephew takes a step backward when you add filler. That's normal. One thing that worked for us was short sessions. Fifteen or twenty minutes of torch time and then a break, because her hands would get tense and everything fell apart when she got tired. Also let him run the machine setup himself once he's comfortable, even if it's just the amperage knob. My daughter dialed it in wrong once, got a ragged mess, turned it down herself and it cleaned right up. That taught her more than anything I said.

Funny how everyone teaches it different. When I got my brother-in-law going on my old Miller, I skipped the no-filler stage entirely and just handed him a few rods on day one. Watching the puddle form and fill is what taught him heat control, because he had to actually do something with it instead of just dragging a bead. He burned through a stack of scrap plate the first weekend but by Sunday he was running beads on 1/8 flat that any farmer would've been happy with. The thing I'd say about your nephew is don't wait too long on the filler. Two things at once sounds scary but kids pick it up fast, and my experience is the dabbing rhythm actually helps steady the travel speed. My brother-in-law said it felt like painting almost, one motion feeding the other. One thing I did differently was let him turn the dials himself after the first session. I told him what the numbers meant and let him crank them up and down to see what happens. Made a mess of a few coupons but he figured out real quick what too hot looks like, and now he sets my machine up better than I do. Ruining your own scrap teaches faster than being handed a preset.

Reading this as the person on the other side of it, sort of. I never had anyone show me, just youtube and a cheap machine and a lot of ugly scrap. What I'd say from that experiance is let him weld something with a purpose early on, even if it's junk. I made the ugliest little charcoal stand you've ever seen and finishing an actual thing taught me more than a dozen practice lines did. Also don't be surprised if he gets cocky after that second bead, I defintely did and got humbled fast. Question for you though, are you starting him on tig right away or is that what you had set up?

One thing I'd add, let him keep the practice pieces. My kid welded a couple plates into a lopsided little bracket that holds nothing, and it's been sitting on his desk for a year. Straight lines on scrap feel like homework, but a finished thing, even a dumb one, gets them hooked. Also the blob at the end of the first bead, everyone does it. Takes a while before the brain learns to lift while the puddle's still hot instead of after.

That point about him watching the arc instead of the puddle, one trick that worked on my nephew was telling him to listen. When his speed was right the arc had a steady sizzle instead of that crackly popping, and once he started going by his ears his beads cleaned up fast. He still asks me what a weld sounds like before he strikes up. Also second the filler taking a step back. Mine got so busy dabbing rod he'd stall out and pile everything into one spot for the first couple tries. Let him burn through a few ugly ones before you say anything, he'll self correct quicker than you think.