We ran into this same problem when we first got Skeeter. He was about 3 months old, and we are both students with jobs, so he was alone a lot! It sounds like you are doing everything we found we could do. Without being there, it's really hard to make sure your kitten is going to get enough stimulation. Chances are, your furbaby is going to sleep most of the time you are away, anyway, and have an active night life.
Something else you might keep in mind: leave blinds pulled up and cords safely tucked away (to prevent accidents) so your kitten can look out the window at all the goings on. Cat's love live entertainment just like us!
One of the most important things you can do is MAKE SURE your kitten gets a lot of attention when you are home. It is suggested that your kitten get at least 20 minutes a day of intense play time. Since your kitten is so young, it is crucial to your relationship together that most of this playtime consist of INTERACTIVE play. Get a wand toy with feathers, a mouse, or a fuzz ball attached to the string and simulate prey movement. You and your baby will be bonding and he'll be cultivating his hunting skills and getting much-needed excercise at the same time.
Oh, also remember that cat's will lose interest in toys that they see all the time. Change it up a bit every day. Put toys away that aren't being used. (We use the carrier as a napping place/toy box for the boys). Pull different toys out every day.
Something else you can do: hide treats around the house so your kitten can hunt for them. This also cultivates their hunting skills and he will be so proud of himself for finding his prizes!
Good luck to you and your little one. Keep us posted on how he does.