Hi,
Have you looked at the placement chart?
http://www.heartofdakota.com/placing-your-child.php Where does your child fall on the placement chart?
I also have a child who was tested at 5 as gifted and who taught herself to read at 3. Sometimes it is hard to know if I am challenging her enough, but I really just try to encourage her natural enthusiasm for things outside of school work as well. She is placed in CTC with extensions. For her reading though, we choose books such as "The Yearling" and books that I read when I was 7th or 8th grade. I plan to have her go through Huck Finn, Tom Sawyer, Treasure Island, and Little Women this upcoming year. She is in 6th grade this year. I find that keeps her very stimulated. Her particular area of being gifted is language arts so that is what I try to keep a bit advanced for her.
I would place him based on the placement chart and then encourage any other desires and learning. My dd loves to explore art so I let her get on forums (with supervision) and she is learning to make digital art without any instruction. She is just getting online and teaching herself. This really stretches her. So I would place with the placement chart and then encourage any natural inclinations that may arise from his being gifted.
She can be lazy with school b/c brick and mortar school never challenged her enough. HOD really has her thinking and not just going through the motions. It has also brought back her love of learning.
As for what to buy at first with limited funds, you need the economy package and the science add-on. If you are going to use HOD LA recommendations, you will need R&S English and the writing instruction for whichever guide he places. I would give the Singapore Math placement test and see where he places. If you have math struggles anyway it might not be bad to switch to Singapore. We ultimately are doing a remedial math year with our dd. She is also about a year behind in math. We chose to just have our dd's use their own Bibles to save money and now I am very happy with that choice. We started off the year with just these things-economy package, science add on, and r&s English with Write with the Best. We have some funds right now so I am adding in the basic package (history), extensions, and DITHOR. This helped us financially and helped our dd's to get adjusted to HOD.
I hope some of this helps.