Hi, with regards the seating issue. The infant can sit on your lap and pay a price of roughly 10% of an adult ticket, or can have a seat of his own, but would pay full child fare ticket (which is often nearly the price of an adult ticket).
However, if you book using an agent and make sure she is in a bassinet seat, your son depending on how big he is would be able to sleep in the bassinet (a small cot which clips into the bulkhead).
If your going down the two seats option, try comparing business class fares. Might work out cheaper to have one business class seat and an infant ticket, and with most business seats they'd be more then enough room for both to sleep much more comfortably then two economy seats.
