I wouldn't get a mortgage on the Portland house, they will require a letter from his current employer stating that he will have continued employment etc (we recently purchased another house and this was required on both of our mortgages since 09), which could raise some red flags at his job. Heloc is an option while he is still employed. Check out a GIS system for your county, they usually will have the school districts outlined so you can know what address boundaries to look in if you decide to rent.
Honestly your best bet is to sell the Seattle house, then use the proceeds to buy the one in Portland, in your offer document you can hinge the purchase of the Portland house on the successful sale of your one in Seattle, giving you an easy out if the Seattle buyers fall through, and it's fairly common to add that clause. Your autistic son will be an issue. I suggest researching school districts in Portland before you start looking/selling. Call or preferably go see the heads of the ones you would be interested in him going to school at. Go meet them, introduce your son to them, explain any behavioral issues etc and clear the air before you put your house on the market, work those trips into his schedule. When you feel comfortable with a certain school, limit your property search to ONLY that district, then sell, buy, move. Your son should already be familiar with the school/leaders so the transition may not be as hard.