Honestly, this should be an excellent opportunity for real learning for your brother. It's a very important thing for a young person to learn that THEY are the responsible party. That a dog **WILL DO** whatever asked to please their humans -- particularly play buddy type humans.
Ask him what might have happened if he'd thrown the ball and it went beyond where he intended and if Apollo had jumped somewhere off the edge of something or had gone into a street and had gotten badly hurt. Is Apollo supposed to know where to stop short? No, it's up to your brother to only throw the ball where it's safe for Apollo to get it.
Handled right, this is a valuable life lesson for your brother. I wouldn't hesitate to make him fully aware of how terribly sore Apollo is **today**. Your brother is getting to an age where judgement is a big fat hairy deal ... and being the one of the group to evaluate an activity and say "You know what ... maybe that's not a good thing to do" is a big sign of maturity.
Those kinds of mature decisions start with learning things like this.
On a practical note -- can you get some chammomile tea? Brew a couple of cups (use 2-3 tea bags to 2 cups of boiling water) strong. But then don't throw away the herbs -- open the bags and dump them in and add the tea and herbs to his dinner (and maybe mix up a couple of snacks with a lot of tea and a little bit of kibble and something yummy like yogurt or peanut butter).
That will help begin to rehydrate him and it will soothe the ache.
You'll also want to massage Apollo -- it you have a Whole Foods near you and can get hold of even some peppermint or wintergreen essential oil that will help a lot with massaging the ache away. But if all you've got is a grocery store or Wal-Mart, pick up some "green alcohol" where the rubbing alcohol is. That has wintergreen IN it and it will help work out some of the kinks. A general all-over massage will likely help do Apollo a world of good.
If you can get to a Whole Foods or a health store open on Sunday that carries Heel products, get Traumeel tablets. Give her one every 3-4 hours (it's a homeopathic and it **rocks** for trauma like this and will go a long way to healing it IF it is simply something small).
As I also mentioned in the "Gracie is broken" post:
Aspirin - you can give aspirin to dogs but be careful -- it's HARD on the stomach and can cause stomach bleeding. 5-16 mg/lb orally (that's aspirin, Ascriptin, Bufferin).
I assume you know but I'm gonna say here because it's a post about "first aid" -- NO acetaminaphen (Tylenol type products) **EVER**. It's a serious poison to a dog and even one regular Tylenol can *kill* a small dog because it damages the kidneys hard, fast and fatally. (Like I said, I know you probably know this but any time we talk about pain, we humans have gotten so hard-wired to reach for Tylenol/acetaminaphen and we don't realize how very different it is from aspirin and dog's just can't take Tylenol at all for any reason.)