I have a 20month old gelding, so I know what its like to have to try to find things for you to do with him. But honestly, there is a lot you can do before you start riding! Getting his respect, and teaching him manners should be your number 1 goal for the time being.
Get him used to pressure, used to picking his feet up, brushing him, and touching him wherever you want to without a response. Get him to do ground work, such as moving from pressure, giving to pressure, going over tarps, ground poles, getting used to "scary things" such as flags, bikes, dogs, cars, tractors, ect, ect.
It will be quite a bit of work, but it is VERY rewarding, and a definite confidence booster!! Just take it one day at a time, and do simple things, keep it short until you both get the hang of things.
As for the 9 year old, if it were me, Id do the same thing with the 23 month old colt. Just take it one day at a time. It should, go much faster with him, granted hes done it before, if not it may take a little longer. Getting respect from the ground will dramatically help things when you get up in the saddle, on either horse!
Good Luck, keep us updated, and Id love to see pics of them!