I realize the last one is a bit of a stretch but it never hurts to ask. The plane needs to be fairly easy to fly, pretty rugged, and have available replacement parts. If I missed one, I am certainly open to suggestions.
Yeah, that Stryker can turn on you, quick! And you need at least a Dx6i to fly it. Maybe a Dx5e could do it, but not very well. (Assuming you have a 5e from the Apprentice RTF?) I wouldn't recommend it for a 2nd plane, especially if he hasn't mastered all aspects of flying.
I've read plenty of people having trouble with the micro Corsair. I would assume there's a risk of a challenge coming with that one for an experienced RC'er.
I would agree with the T-28 being the best choice of the bunch, perhaps the P-51 isn't far behind, but keep in mind, I've only flown someone's micro T-28 once, and never even seen that P-51. I'd look for the opinions of those who've flown both.
Ultra micros are actually a little harder to fly than you might think. Generally, the bigger the plane, the more slowly it reacts and the easier it is to fly, so ultra micros can actually be pretty twitchy.
I threw the Stryker in only because he thinks "it's looks cool" and I was fairly sure that it would be beyond his ability at this point. Several posts on this thread confirmed that so thank you.
I did find it interesting that the T-28 garnered so much support. It will be the next plane I get to replace the Apprentice due to the overwhelming positive comments as far as a second plane. It's great to know that those same flying traits have been passed on down to the UM T-28.