I have no problem with the example as such. It looks very much like a series of fragmentary utterances alternated with some unexpressed thoughts that can be easily (imo) reconstructed, something like:
'I don't get it... (I have invested so much time/money/effort in this party..) with all the balloons and the hats.... and all that stuff..(because I was sure he would like it) It must have been a lot of fun (that's what all boys of his age usually like) (what do you think?) Right? (if my reasoning was right then I really don't understand) how come my son won't speak to me (even worse than that) he's acting like I've done something wrong'
If this piece sounds convincing, then I'd say what makes the poor father suffer is actually the clash between what he was sure was the right thing to do and the reaction of his son that he is struggling to understand. If so, then here 'must' expresses 'certainty based on some, quite possibly wrong, yet logical deduction'.
ps. Funnily enough, I was sure it should have been the boy's father speaking (wrong assumptions ) Clearly, it could have been his mother as well.