No, he will not be able to see your picture mail, the most he can see is the itemized list which shows that you recieved an MMS message and from that number but wont be able to see the content, same goes for plain old text messaged
Your phone upgrade will still be every 2 years or whatever it is with sprint, but you loose the ability to early upgrade as your phones not the primary account, but if you sweet talk them usualy you can get them to take care of it anyways
As with me, when i got my upgrade to my droid I was 3 months from my upgrade date, if my line was the primary account I coudl ahve walked in the verizon store and they would have done it no issue, but because my line is not the primary I had to call verizon and sweet talk them.