2 yr old/constant screaming and hurting himself

my son is 2 1/2 yrs old,diagnosed with global dev delay,he goes through these spells of screaming and constantly hurting himself and anyone around him,he has turned real mean,they have him on risperdal,lorazapham,and they have been giving him haldol in the hospital since they admitted him on saturday,none of these seem to be working on him,it might put him to sleep for an hour then he will wake right back up in the same screaming condition where u cant do anything but restrain him from getting hurt,these spells started of minor and would seem to go away for months at a time where there is no screaming and he enjoys love and doing things,then when they  tend tocome back they seem to get worse,this time i feel like i have completely lost him ,his spells dont stop 24/7 all day and night long,i know theres something mentally going on in his brain,but i just dont understand why these spells would stop for months at a time and  he comes back a loving little boy again then out of the blue the spells come back,his neuroligist says in his 25 yrs he has never seen a 2 yr old with this kind of behavior,he had to do a special authorazation just to get the risperdal approved since they dont usually give it to kids under the age of 5.my son does not talk yet so its not like he can tell me,theres no tears when hes crying so i dont think it could be pain,do u have any explanation to why these spells seem to come and go.hes currently admitted in childrens hospital right now due to these behaviors and  docs are trying to get a metabolic specialist involved so they can run some tests.