Not backpacking, but we just got back from our 7mo olds first camping trip. I was vehemently against her sleeping in the bags with us for fear of SIDS(suffocating or crushing) but somehow my wife managed to get her into bed with her...
Anyways, it worked out, I think her mothering instincts kept her from rolling around or moving at all with the kid in the bed. I on the otherhand do not have that awarness, so baby slept on the OTHER side of mom. We were under unzipped bags though, blanket style.
Originally, the plan was to put her in a legless onesie, then a legged and footed fleece onesie, then a thin jacket, then inside a down bunting. Of course, the layers need to be adjusted to the temp, which is the hardest part because you have no idea how warm those are unless you are also wereing the same type of clothes.
Regardless of which direction you take, be aware that at that age he will be rolling around and wiggling A LOT.
All that being said, I would opt for a quilt and have the baby sleep next to mom but away from you. IF you think mom can trust herself to be aware of baby. It's a risk you have to weigh yourselves. Suffocation/crushing vs. Overheating/freezing.
Fair warning, they eat whatever they can get their hands on...