I'll be very honest with you, as we have the same situation in my family. My parents have permanent guardianship over my two nephews. YOu do not now how many times, his 'parents' have 'cleaned' up their act, are really on the right path start coming over and FINALLY being involved with their kids, only to have a relapse. Think about how much fun it is, and how excited they were, and then all of a sudden have it stop, no rhyme or reason at all (in the kids minds anyway).
Let me tell you how emotionally trying, dramatic and unstable these kids have become. My parents finally had to call it like it was. The kids started acting out, it was very troubling to watch and be apart of. Basically all visitations have been halted unless supervised and drug tests have been passed (this was court ordered from the beginning but never carried through)
Now granted they never said they could do something and then not follow through with it, but I'm sure the guardians of this little boy are taking it very slowly, which they should. Now I'm not saying this boys parents are going to relapse or that they have had repeated offenses, but let me tell you, you can't be so sure with previous drug users. As much as this little boy was so excited, think of how much of a low if it becomes a pattern then just stops.
Guardians are probably just protecting him in some way. If he was anything like my older nephew even though he had fun with 'mom n dad when he got home he was a bare to deal with. It was just a emotional sense of uncertainty that he couldn't handle on his own (granted he's only 7).