L.,
I am putting you and your son in my prayers. This must be very difficult for you to deal with, having so much to sort through to provide what is best for your son.
I personally have Lyme's myself and was first diagnosed when I was only 18. I am 35 and have only had one other flare up in that time. I have been very lucky. It was believed that I had contracted the disease almost a year before I was diagnosed. That being said, I was treated with a high does of antibiotics for several days at that time. Your son can be treated for the Lyme's in much the same fashion.
As for the PSRA, I am not familiar with that but did some research and found that this occurs after an unusual reaction to strep throat. It is considered another form of rheumatic fever and this occurs when there is already an issue with one's immune system being compromised in some way. Anti-inflammatory treatments are not very effective, though I would have to say that natural treatments for this can be found.
Based on what I know, and personal experience, your goal right now would be to get your son on an acute antibiotic treatment. One suggested for the PSRA is a single shot of 1,200,000 units of benzathine penicillin which will eradicate the bacteria and gives protection for three to four weeks. As for the Lyme's that very well could be treated with that, but only a Dr. could tell you that for sure.
Aside from that, I strongly urge you to get a second opinion from someone who specializes in the treatment of PSRA and also Lyme's Disease. Long term antibiotics can do irreparable damage to one's system if not countered with the correct nutritional support. So, before you even get that far, get your son on a good pre and pro-biotic as suggested by several other moms here. Considering the PSRA, your son's immune system is already compromised and the Lyme's can spread rapidly because of that. Build back up his gut (50-80% of your immune systems defenses are found here) and put him on a path to recovery even before you seek intense treatments for him. Be cautions with the timing of the pro-biotics and the administering of the antibiotics. They can counteract each other. This is easily done by waiting a good hour or two after the meds before giving him the pro-biotics. Kefir is a wonderful source (as an additional source) of these and can easily be mixed into delicious fresh fruit shakes. There are many excellent brands of pro-biotics to choose from. I can help you find one that would suit your son, if you'd like. Others have made excellent suggestions already.
I wish you the best in treating your son, and pray for his recovery.
Blessings,
Rolinda
Wife, Mother, Friend