Well, I ran a Swampert/Lucario deck for regionals last year and... frankly did badly since I didn't account for the metagame (...No one plays Crystal Beach in Washington; why do they play it in Oregon?! D=).
I found a HEAVY reliance on Scramble Energy, but those are gone now... Hm...
I DID like the flexibility though once you got energies onto the table, so let's see what we can do here...
Finding a way to throw down energies and getting them to stick would be useful. Regirock would help out Lucario, yes. Making sure your Lucario lines are something like 2 Lucario DP, 1 Lucario LA, and 1 Lucario MT (LV.X) would help too, to give you the most flexibility.
I think Unown Qs and Mantines are probably going to be highly useful, so you can keep energies in play as long as possible. I guess we now have Energy Patch too, but you don't want to live your life on coin flips (especially with kids, since it seems most of them are rather bad with coins; not that I'm trying to stereotype them).
I guess that IS the most important point to keep in mind with Lucario/Swampert: Keep your energies in play to be able to attack every turn. IIRC, Juniors tends to be more "Who can keep cards on the fields" than "Who can deal >9000 damage ASAP", so as long as your brother understands Warp Points (...MAYBE Metagross, but that sounds a bit much), remembers to draw cards for his turn (...AMAZING how many people forget that <_<), and understands how to reduce a pokemon's remaining HP to 80 (for KO range by Swampert) or 90 (KO via Swampert if you play Buck's Training/PlusPowers), then he'll be fine.
If your brother's willing to try more risky cards, then Cresselia Lv.X, Metagross, Spiritomb, Forretress, and Frosslass all add interesting aspects to this deck, but more chances for it to go wrong (...Well, Frosslass actually seems pretty good in Juniors because kids tend to throw every pokemon they see onto the table, leading for easy spread KOs <_<; again, not being judgmental here ;P).
'Bout it I can say without actually trying the combo out in DP-on. I realize there's quite a bit of LA cards involved as suggestions, but it shouldn't be too hard to obtain copies of (You'd only need ~2 Regirock, for instance, and Buck's + Metagross are easy to find in Starter Decks).