If he's walking and bearing weight on it, it shouldn't be cause for rejection at slaughter time. It would only be if he wasn't able to walk or move without being in obvious, severe pain. A bit of a limp wouldn't make a difference IMHO.
You could give an anti-inflamitory - but if there's no heat it might not make a difference and you'd also then have a potential delay because of withdrawal times for entering the food chain.
It's entirely possible that the back leg has been damaged

But, the best method to catch piglets is to run your hands down their body (as they run between your legs) and clasp and lift both back legs together - we do this and then transfer them to a 'hold' (lying across our chests with a firm hold on at least one leg) to take them out to the car/crate/trailer - this is assuming they're still small enough to easily carry without damage to us or them, if they're too big they get walked (or gently herded) into a trailer.
HTH
Karen