Congratulations! May God bless you and your son as you grow together as family. We, too, have been blessed to receive children from the Lord via adoption; may your rewards be as many as ours.
Your instincts to begin at his current level until he catches up are right on the money. Following this approach in all subjects is generally the least stressful, and therefore the most productive, method of attack.
In regards to his writing skills, you might use Jensen's Format Writing [available through CHC] to help him understand and practice the building blocks of composition. [While he is developing writing skills, you might overlook spelling and grammatical errors in his writing, focusing instead on writing in a coherent and logical manner. Grammar and spelling can by addressed in their respective classes.]
A few more ideas:
Make careful note of all that he does well, and each improvement he makes; praise each accomplishment, and each gain.
Saxon's incremental development is ideally suited to skipping every other problem--not every other lesson--if he is doing well. This cuts each lesson by half, allowing you to cover two lessons per class [as long as he is doing well and understanding each new concept] and thus completing the book far more rapidly. If he is struggling, of course, stick with one lesson per day. Of course, be certain to cover the new concept introduced at the beginning of each lesson.
Try to set aside some time in the evening for oral reading together. You might select adventuresome historical novels or biographies to cultivate a love for reading while bolstering his knowledge of history.
A variety of presentations helps keep academics from becoming humdrum. Computer software and free online programs offer geography, spelling, and math games for painless tutoring.
Remember that your son doesn't necessarily have to graduate at a certain age. We had a fifteen year old foster son [from a foreign country] whom we enrolled in sixth grade based upon his then-current skills. When he was placed in an adoptive home, his family and school district continued to allow him to advance out of sync with his age. He graduated high school at age twenty, earned at least one college degree, and is a valued and productive member of his community.
May God bless and guide your family!