As for the interior speakers, If they are the same as mine, it is a simple swap. My covers simply snap on and off, and there are 4 screws underneath holding them in place.
You must measure the diameter of the mounting hole, and find speakers the same size.
(typically 5 1/4" or 6 1/2" are the standard sizes) and be sure the depth of it will fit in your opening. The back of the speaker will be labeled with its Ohm rating, and you should find speakers of the same rating (typically they will be 8ohm or 4 ohm). All the speakers you look for will have this information listed, and hooking them up is a simple 2 wire connection.
A great place to look for them would be partsexpress.com they have a wide range of brands and sizes, and the prices are great as well. Nearly any replacement speaker you get will sound better than what they put in from the factory.
If you don't feel comfortable doing the research, why not just get him a gift card from partsexpress or someplace similar, and let him handle it. Either way, I'm sure he would be pleased.
Also, as stated above, the exterior speakers must be of the Marine variety.
2010 sunset trail ST26RK, 2012 Nissan Titan SV crew cab, equalizer hitch