I've nursed people with hepatitis and AIDS and frankly I think we've gone too far the other way in our approach to these fatal illnesses (and they ARE still eventually fatal even though if your body tolerates the treatment - which not everyone does they're strong drugs - you COULD live to a similar age that you would have without them it is highly likely that's what will kill you and they are both horrendous illnesses).
I've NEVER had unprotected sex in a relationship without first establishing both have a clean bill of health and that includes my ex husband who was a bit of an arse about it!
Sti's are on the rise because of complacency. Also they are often asymptomatic in men, sometimes asymptomatic in women but less so (we really draw the short straw in reproductive and urinary health!) plus cliche cos it's true - men are shit at going to dr especially if it's something affecting their manhood!
The 'snooping' in comparison to his actions is minor. Ask him why he's ordered the test, point out its not advised to test this way because it's
A - a huge deal whatever the result and people need support in coping with the result, even if it's negative.
B - home testing especially of blood is notoriously inaccurate.
C - personally I think sending potential bio hazard through the Royal Mail or whatever is fucking irresponsible!
D - he could get a positive result and do nothing about it, not get support, treatment, put new partners at risk.
E - as pp have said this is only ONE condition being tested for, a full screening at a sexual health clinic is far more comprehensive and accurate. Especially as there are new antibiotic resistant strains of the 'less serious' sti's coming through AND the consequences of sti's even if you're lucky enough to not get a fatal one include infertility, cancer, heart disease, brain or nerve disease etc
Frankly it doesn't sound to me like either of you are mature or sensible enough to be in a sexual relationship. My 15 yr old dd has more of a clue!
Ask him properly and get tested at a clinic asap and start using condoms! And talk to potential partners about their sexual/health history (remember things like iv drug use, blood transfusions in some countries etc are also risky).