There's a few things he could've done differently.
You'll notice that he's retreating diagonally at full speed towards our left hand side corner post, that's not how you're taught to defend in a 1 on 1 situation at Fullback. In these scenarios you're taught to play either A) passive and jogging (so that you're in a position to match his lateral movement, but still in a position to accelerate once he commits to his line) or B) aggressive and squared up (get yourself between him and the goal line and then close the gap between yourself and the attacker, forcing him to make a much faster decision than he's anticipating).
If you're retreating at full speed diagonally with a guy running at full pace towards the goal line, you've already let them score. Instead of trying to sprint diagonally towards where he perceived Butcher would be if he maintained a straight line, he should have run SLOWER (yes, I know this is a bizarre thing to request from Boyd) and squared his body up with Butcher once he committed to a line.
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At the point where Butcher makes his step on the inside, Boyd is in between him and the goal line, which is good, however because he was over-committing to the line that Butcher was initially running, he has no possible way to defend any sort of step to the left. I can only assume that Boyd was going at full speed because it's significantly more embarrassing for a Fullback to get outpaced by a forward than it is to get stepped by one.
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As you'll see here, he's only about 2-3 metres away from Butcher at this point in time, and that's well after Butcher has made his step and Boyd has overrun it. If he was more patient, he would've been at least within arms reach of Butcher at this point and able to attempt a tackle, which is obviously better than not laying a finger on him.
TL;DR - Boyd was over-committing to the line that Butcher was running, which made Butcher's decision incredibly easy to take a step back to the left and score untouched. There's many Fullback's in the NRL who would of at the very least been in a position to attempt a tackle in this exact scenario. (I'm not suggesting that Boyd
should have stopped him here, it's a very difficult scenario with a barnstorming 2nd rower running at you, but at the very least he should have been able to lay a hand on him)