I really want to know an update on this issue after all these years. I am facing a similar situation with my 10 year old daughter these days, and talking to her, or explaining her, doesn't work. There is nothing that can stop her once she starts to laugh and its becoming stronger and unctrollable every day.
If you get back on this site please update what happened. my 20 month old is concerning me with this too and not sure what I should do.
My 2 years old son too has autism and SPD and also has these laughing fits but I have a hunch that there may be something more neurological going on. I read online about gelastic and dacrystic seizures and from what I understand these can be really scary for kids even though they appear to be having a good laugh. Try this link http://www.epilepsyfoundation.org/aboutepilepsy/seizures/Gelastic-and-Dacrystic-Seizures.cfm and see what you think about them. I'm having my son go in for an EEG at the end of the month to test this for these seizures. I hope this is not the case and that he's just really happy but I can't help worry that he might be frightened and I've been laughing "with" him all these times ...