Girl go get him and bring him home.
Siamese are my other weakness. I have rescued many and they are so loving. Its as though they know you saved their life. Alot of times it takes longer for older cats to bond to each other, so dont worry if it doesn't happen instantly. You didn't instantly fall in love with your husband did you? Give them time and space to get to know each other. Most likely they will grow on each other. IN my experience the only time I have had a neutered cat to spray is when I had an unspayed female there in heat. Keep in mind you would be saving this guys life. If I was close I would snatch him up in a heartbeat. As far as declawing I think that is a personal decision. Its not as bad as most people think. I have worked at a vets office for many years and it doesn't tramatise the cat like most people think. Is it painfull? SURE! So are many other things we do to them. But it does heal and most of the time the owners are thankful they did it. I guess bottom line is.....This is your decision and you have to decide if you want to bring another life into your home. But I know this little guy would sure love you for it...........
As far as being gone next weekend you could use that time to board him at your vets for his shots and what ever other vet treatment he will be needing..
Just my two cents....