My background is exactly the opposite - no sidecar experience (other than as a passenger for a couple of short (UK) trips but I've suffered the trials and tribulations of trying to go places with young children (who are now, fortunately, a bit older).

If your son is 10 months old now he'll be obliged to start school after Easter 2019. Presumably you'll be looking at a summer trip to Mongolia so that'll be summer 2018, when he'll be 4 1/2. Based on what we were doing with our two at that age you might (just) get an hour or two of good will out of a child that age before they get bored and want to get out. Interesting scenery or anything that would keep adults occupied will be as nothing if he is warm, cold, hungry, thirsty, bored or just wants his mother.

And that's based on my placid easy going son. My daughter would not entertain a sidecar trip for more than about 15 secs as she would feel abandoned.

A couple of months of "acclimatisation" beforehand (going to Tesco's etc) might give you some good will through familiarisation but I wouldn't be surprised in our current climate to find "concerned (UK) citizens" expressing their opinion that a three year old in a sidecar is a child welfare issue. Not my opinion as I think kids are more robust than we give them credit for but they certainly have different priorities at that age.

Sorry to say it but this is one occasion where being close to mum and dad is going to matter. At that age some tlc in a TLC will make for an easier and more enjoyable trip for everybody.