Because you have an older daughter, you will want to have a small item or two for her sake - maybe something from Santa and something from the family. Is there a practical item that he might be able to use? What about a rolling duffle/suitcase? There are some really cute personalized ones available and it's something he could use for years. At this age he won't care or know if he got fun toys. How about a music cd - fun kidsongs or classical are both great for setting the tone of playtime or sleepytime. If he doesn't have one, you could get a cd player to put in his room to play the music on while he falls asleep... and he can use it for many years. You could start a tradition of giving them each a special book or holiday ornament each year. They will love to dig out their special ornament and hang it on a special place on the tree each year. Otherwise, I'd put any more money that you want to spend (if you are concerned about keeping things even) into a savings/investment account for when he really needs something - get that college fund started now.
I have 3 close in age - 5,3.5 & 18 months. There is nothing wrong with the toys that the first or first 2 played with - no need spend money or collect things just to do so. After collecting way too much each year from a wide circle of extended family, we are burried in toys and gear. I am despritely trying to keep things simple this year. My 18 month old has practically anything she might want from the older 2 kids. She will be overwelmed with other activities going on so I'm sticking to the functional rule primarily for her - a duffle suitcase (my older 2 have them already and they have gotten great use out of them - the perfect size for toddler/preschool packing even for a week trip to CO!) and a personalized hooded towel/robe (my older 2 have them and they are wonderful in the colder months to keep them covered since the towels slide off them and she's outgrowing the baby style). While they might not sound exciting, I know she'll love dragging the suitcase around the house for a couple days and playing dress up in the towel when she opens it. She'll want to take the older kids' toys anyways once the initial excitement passes.
Try not to worry too much about it - just enjoy him ... He will love the lights and sounds and smells that are all around during the holidays. That is how a baby learns - let him enjoy those and don't be concerned that you are giving more to your daughter and leaving him out - he'll have plenty of years to want fancy items.