I don't have any boys and my one daughter is 18 months, so I am not exactly an expert in this area but here is what I would do if I were in your shoes.
First I would watch him for a couple of days or a week and try and see what he's interested in (you probably already know quite well)You didn't say how old he was so maybe if he's really into trucks, cars, airplanes, trains etc... you could buy some magazines help him cut out pictures of them (maybe if he's quite young this would also be a teaching time of how to use scissors). Get a big binder with card stock paper that he can glue his pictures in and get dividers for each different mode of transportation.
Maybe he would like dinosaurs, get tons of stuff on dinosaurs, or snakes, or bugs. If it's bugs, get him a bug collector and then you can collect bugs. There are places you can buy larva, and watch it grow to a caterpillar and then into a butterfly, what a wonderful learning experience. You can also buy an ant farm, gold fish (this will teach responsibility as he learns to feed it, and wash the tank with help)
Hope some of those ideas help.
Also, with the gentleness things... I am convinced that kids learn stuff like this by example and obviously instruction. Make sure you teach him to always be gentle to you as you are a Lady and every other Lady out there, Grandma, nieces, etc... Try and teach him chivalry. Read him books about it, like the midevil times, opening doors for you and other ladies, the role he has as a little man being the protector, especially since you said you are a single mom, I think he will think that is really cool and even if he is young like 2-3 (again not sure) I think he will grasp the idea) Buy him a sword and let him be your protector :-) I think he'll like that.
That wasn't much on gentleness, but if you teach him that it is fine to play and wrestle with his little brother but he's still little and needs love and care as well.
Gentle with mommy, wrestle with brother...
Hope I helped a little