Another vote for the 125 2 stroke. The YZ is probably the best choice, but realistically, any brand will do just fine - condition matters more than brand. I am the father of a 15 year old who is just learning how to ride, and I got him a TTR 125 to develop his skills. I have a 2006 TM 125 waiting for him for when he gains the confidence and skills to move up to it. The upside for me is that I have fun riding the 125 as well! Consider the weight of these bikes - the 125 2 stroke it is right around 200 lbs, which is not that different from your TTR 125. The TTR 230 is around 250 lbs. That is a massive difference - you'll certainly feel it when you drop the bike and have to pick it up. The TTR 230 has about half the horsepower of a 125 mx bike, and much softer suspension (and only 36 mm forks, which on a bike of that weight, does not make for very precise handling). Myself, I ride a new KTM 250 sxf and won't let my son touch it until he has mastered the TM. The 250f isn't as heavy as the TTR 230 (it falls about halfway between the TTR 230 and a 125 mx bike), but it feels much heavier than the 125 when you're riding it, and is much more powerful. I think that many here will also agree that the best thing about a 125 2 stroke is the smile on your face when you're riding it!
All of the above being said, if you're just going to be trail riding, get the TTR 230 (unless you can find a KDX 200 in good condition - awesome trail bike!). That is what it's made for. And like others have said, if your dad is buying it, be happy for whatever he gets you!
Current rides: 2021 TM 144