This sounds improbable although not impossible
The US has some quite tight regulations about this sort of thing even 7 years ago this was the case. I'd think you probably have to go some way out of your way to find US websites containing this sort of material.
The question then arises how he'd know that the model was over 16 and not 18 unless the US authorities were involved in raiding the site.
This is a perfect example of the sort of case that the so called 'Sarah's law' was rolled out for.
http://www.independen...lled-out-2040992.html
I would check with police if I were you.
Interesting that he's told you specifically that it was 7 years ago. - a prison sentence for less than 6 months requires 7 years notification on the sex offenders register.
Also If you google sentence indecent images you'll see that these days (although not necessarilly 7 years ago) this offence does not normally attract a custodial sentence on it's own - especially if you believe the mitigating cirumstances.
All in all I'd say there are a few details of this story that sound odd and whilst that could just be because of the passage of time, I'd not accept the story at face value if I were you