From the sounds of it, I'm home a lot more than you, but in part got Jazz for the same reasons you mention. When i was home, Otis was ALWAYS under my feet, asking to play, crying constantly for my attention, and making me feel guilty that I couldn't be a stay-at-home mom to my kitty. I had a feeling Otis was social (he liked strangers and new things, never showed any signs of being timid, and never demonstrated any signs of aggression when i can home smelling of a strange cat - which happened a lot since my parents have one...) and getting Jasmine was the best thing I ever did. Otis still wants his mom, but is not nearly as vocal, and is often playing with his sister when he used to be sitting at my feet crying. I'm not saying it will work in all circumstances, but in mine it was definately the right thing to do - I would say Otis is happier.
As for male/female and kitten/older - how active is your cat? If he is docile/sedentary, he may not have the patience for a rambuncious kitten and could just get frustrated. On the other hand, if he likes to play, a kitten may help the matter (or make it worse as there would be two of them running around...) I guess bottom line is you know your cat and his activity level/personality, so that's a call you have to make. If your cat is dominant, i would recomend a female, however - adding a second male may make your cat feel even more threatened.
If you do decide to do it, I would seriously recommend making sure you're home more than one a hour a day during the introduction period - I know I found it too kconsiderably more time than i had thought to give both Jazz enough attention that she felt comfortable with me and Otis enough that he did not feel neglected/second best....
Getting the second cat was the best thing I ever did - it gave Otis a friend, Jazz a second chance in a loving home, and me a second being who loves me unconditionally... But it may not be the right choice for everyone.