Upvote:6
Your boyfriend most definitely may not stay for most of the time in Belgium or use a Spanish visa to circumvent the limit on short stays in the Schengen area. Each country in the area retains the right to regulate long stays by non-EU citizens, a Spanish visa does not give you permission to live in Belgium. That much is clear, quite apart from the details of how days are counted and whether or not you're likely to be found out.
Some details on your specific questions:
There is no general βfree travelβ for non-EU citizens, the rules are more complicated than that. And the lack of border control only means it's somewhat easier to flout the rules. Schengen countries consider this risk acceptable (borders weren't strongly enforced before either) but it does not mean your boyfriend is allowed to stay as long as he wants in Belgium on a Spanish visa.
The rules are not crystal clear but not for the reason you suspect. The time spent in the Schengen area before he got a Spanish visa was long enough in the past that a new 90-day stay would be permitted. Those days need not concern him and if he had been in Spain since October, there would be no issue. The problem is that if you count all the time spent in Belgium (an interpretation a border guard is likely to take), he has already overstayed and certainly isn't allowed to stay even 9 days.