Oh, man! 10 months old, 7 weeks means he'll be... 17 months when the new one arrives (barring excitement)...
1. At 17months, the biggest challenge will be having two small people who have no way of understanding what 'wait' means. All of their needs are needs (not wants or demands) and all of them are immediate. For your older, this will mean that he spends some time before he is emotionally- or mentally-ready, being 'put off' because the baby needs something. If you can hire or enlist household help so as little of the running of the house is your problem, you will find you have more time (and energy) to meet your children's needs. Also, with only an emerging sense of himself, your older lad will quickly forget what it was like to be the 'only'.
2. Happily, that will (should) pass before your lad is a terribly-active 14mo. Make one room in your house a 'safe' room, where you can lie down and rest knowing there are only safe toys (stuffed things, board books), no health or safety hazards (like drapery cords, windows to fall out, uncovered outlets, etc.) and soft furnishings on or close to the floor -- a mattress on the floor is safer to fall off of than a bed of any height. Many women report nausea being greatly minimized when they have a way of getting enough rest.
3. Your body has barely recovered from the last pregnancy, so your muscles will distend rapidly (you'll look a lot 'more pregnant' much faster this time), but if you're in relatively good shape, shaping up again will not take any longer, provided you eat well and make sure that rest is constantly a high priority.
4. Probably just the talking you do about the things you're arranging, the appointments he accompanies you to, and the shopping you're doing, as well as being around the people you're talking about it in general with, will suffice. He won't be very verbal, and will still have no concept of 'different later' -- that doesn't really work for kids' brains until they're around 3. His life will change suddenly and he won't know (or really even care) why.
Tips: get the book 'Mother Nurture' and start putting the main advice into practice as soon as possible. Your body is recouperating from a pregnancy while it is caring for a baby and growing another one -- you have a lot of physical and emotional demands to rejuvenate yourself from...