I wouldn't worry about the physical development of an 11 year old. Eat well, train well and teach him good fundamentals, you will see how the rest shakes out in 2-3 years.
Also if your older son got to 6"1 190 why do you think your little kid will stay small?
Like most HS kids he probably will need to gain some weight but I wouldn't worry about how his height develops.
If he really stays short he needs to be a good hitter, have some speed and arm to play all over the infield and have at least enough strength to hit it consistently out of the infield and ideally into the gaps for doubles, maybe even some homers.
Hitting is always number 1, if you can hit for average you have some baseline usefulness for the coach and if you add some power that's even better.
You also need good legs because at well under 6 ft you are unlikely to play 1b or DH and ideally an arm that allows you to play other infield positions than second base.
Although there are also exceptions to this, temarr Johnson is short with a fringy arm and defense and he got drafted top 5 because he has excellent hit tool and good power for his height. But ideally you have the arm for all infield spots.
In short he should develop all his tools, hit, speed, arm, fielding and some strength/power - but really that is something that applies to almost every player.
Sure it probably isn't the best idea to make him 5'9, 210 but that's not super likely to happen anyway so just focus on getting more athletic and a better ball player.