My 3 month old usually sits in the rear with car seat on isofix base. We regularly do long journeys and if his dummy falls out the front passenger reaches over and pop it back in (he usually will scream if we don't).
However, I soon need to do this long journey by myself with no other passenger other than DS. Would you put him in front beside you (obviously airbag off but no isofix so car seat fixed with seatbelt), or leave him in the usual backseat? He is most likely to scream when stopped at lights or in traffic so if he is next to me I'm thinking it would be possible to pop his dummy in, and he will be able to hear radio better (which also soothes him).
Journey is 6 hours plus rest breaks.