Hey Albert,
I was told my cancer was very agressive too, it turns out that's actually a good thing because agressive cancers are generally easier to kill with chemo (I hope that told you about this). Chemo stops cells from multiplying and if they're short-lived cells (which the fast growing ones usually are), the tumour will be more affected.
Those odds are pretty good. I was given a 99% chance of mine returning without followup treatment and 10% following treatment (I was Stage IIC yolk sac tumour). They may not tell you the positive aspects of the prognosis until after treatment, because some people don't take it seriously. You'd be surprised about how many people refuse to have treatment.
Anyway to answer your questions:
1)as far as I know your results will be stored with whatever hospital you're at, and if you change hospitals they send the file to the new one. The doctors in the public system that I was in could access patient information through a database from any of the public hospitals in the region (and possibly state). Not sure about the transfer from private to public though sorry.
2)I think you can choose whatever hospital you want, but they have to accept you and you'll need a referral from your GP.
3) I had BEP chemo, which is not usually administered for seminomas from memory. It was pretty full on - let me know if he's having that and I'll give you some info. I was told before we found out that it was a non-seminoma that treatment would either involve chemo or radiotherapy. The chemo would involve a day-visit once every 2-3 weeks, for 3-6 months, and that I'd be able to continue working during it, oh and that my hair wouldn't fall out. Radition treatment was meant to be similar (although only something like 3-6 sessions - sorry, I can't remember the details well).
I had a lot of side effects from chemo, but your brother probably won't have that type of chemo. Let me know what they're going to give him. The main tips for getting through chemo are - get some good books/ipad/laptop or other entertainment, it involves a lot of sitting around.
If he's getting strong chemo (talk to the oncologist about this): Take anti-emetics/painkillers like clockwork - I actually set alarms for myself to remind me to take them on time. Even taking something half an hour late can mess up your day. Definately take everything they give you, even if your feeling good. If he's perscribed dexamethasone (a steroid that helps with nausea), he'll need sleeping tablets for at least a week after he stops taking it (no-one told me about that).
Let us know how he goes.
Steve