He looks a lot like Kenzie did when she came to us in terms of height and underdevelopment so I don't doubt that he really is 2 years old. Poor guy.
Make sure that you quarantine him away from the other horses, especially unvaccinated ones, for at least a few weeks. One, to keep him from spreading worms, and two, because you never know what he might be carrying. Kenzie looked very malnourished when we got her so we assumed her lethargic demeanor was from that, but within 24 hours of coming to our farm she was on the ground, unable to get up, unable to poop, spasming, with a high fever...and she was diagnosed with HIGHLY contagious Equine Herpes Virus a day or two later. Imagine if we'd let her have contact with one of our other 40 horses? Only some of them are vaccinated for ERV, and sometimes that vaccination doesn't even work. Our entire farm had to be on lockdown for 30 days.