I would only feed him one rat for his first feeding even if it is small and even if he wants it. Go slow. If he does well, you can feed him another in 10 days. It will not hurt him to go slow. Since he may not be used to rats, you don't want him to regurge - that will do more harm.

Keep in mind that boas are NOT ball pythons. They have a much slower metabolism and do better on the "less is more" feeding philosophy.

If the UTH will maintain your temps properly on it's own, then that is all you'll need. Boas do well with belly heat.