If you've taken him already you'll know that even if you feel warm, he most definitely won't, and extra layers are a must. Ben Lomond seems fine if it must be a Munro, but so many people climb it, you may find it very slow as everyone will want to stop and say hello with him in the back pack. Make the most of the time he is relaltively immobile, because climbing a hill with a mobile small child takes about four times as long.
If it doesn't have to be a Munro, how about Ben A'an. Lots of fairly small kids do this, so will give him something to look at, and the view is fabulous. This wee guy we met up there must be well into his teens by now.
5 george aged 4 plus by
Margaret Squires, on Flickr