I was in a similar situation when my daughter was a baby and I babysat my nephew who was only 1 yr old at the time and it was tough keeping them both occupied since my baby was such a high-needs child!
I'm not good with keeping to schedules myself, but here are some suggestions so that you can feel a bit more "hands-on" with your son too!
Since he is quite a bit older than your daughter, there are lots of things he can do without you constantly supervising. For example- if you make a dress-up trunk, he can play dress-up and come and surprise you with costumes and you can play pretend with him while still holding the baby.
Give him some play-dough or water color paint (at the kitchen table!) and give him things to make/paint, etc. Maybe you could make a theme week? For example- this week will be dinosaur/pirate/birthday week and we will dress up and play games and paint, etc. about our theme. (You could even have him start to trace letters, etc. There are LOADS of websites with printable worksheets that are even themed!)
What about playing games that require no hands-on? Sit by the window and guess the next color car to come by, or play "I spy" with colors, shapes, etc. that you see in a room.
If you have a yard and want to be outdoors some, send him to collect sticks/branches and you can lay them out in shapes to make pictures or letters... or a scavenger hunt- tell him "bring back 3 red leaves" which is good practice in listening skills/directions. My daughter is 6 now and LOVES to "paint" with a pail of water and an old clean paintbrush (works well on porch or patio, outdoor furniture, etc)
If you don't mind a little mess, get a shallow pan or rubbermaid container and fill with dry rice and/or beans and let him play on the floor or at the table with it like a sandbox- use sifters, measuring cups, etc.
My daughter also loves to try picking things up with kitchen tongs, or clipping clothespins to things, playing with ice-cube trays, etc. -- look around your house for safe alternatives to regular "toys" to entertain him too.
There are also good computer games for his age- check Walmart and Marshalls for some cheapies but goodies! And don't forget that websites have free games- try www.pbskids.org for some educational games with characters he will recognize.
As far as having to hold the baby all day, two things worked really well for me- try a sling (walmart sells these, as does La Leche League) which is a piece of fabric that wraps around you diagonally and holds your baby tight to your chest and leaves your hands free. My daughter LOVED the baby swing too... you can find these at thrift/second hand stores sometimes for just a few dollars, or maybe $20 at walmart for the wind-up kind.
If you would like ideas for websites, email me (____@____.com) and I can send you a HUGE list I have made for my daughter. I began homeschooling her when she was 4 and have loads of resources for that age group!
Remember, you have only been a mommy to TWO children for a month- don't be so hard on yourself; you will find your groove! Just give it time and keep trying!
Hope this helps,
Amanda