How do I resolve this?

I was due to go abroad with my son who is 13 and has a diagnosis of ASD. The day before our flight I was packing and checking everything was in order when I discovered his passport was missing. I knew without question that it had been in the same place for the last week plus but now it was nowhere to be found. I ripped my bedroom apart, searched my whole house and asked my son if he knew where it was. Told me he didn't know, hadn't seen it and hadn't even known where it was. 

Other family members came and searched my house, top to bottom. We searched in the sheds outside, in the garden and any other place we could think of. I was sobbing, begging him to tell me if he knew anything and he still said he didn't. 

Having missed the flight and not able to get a replacement passport for at least a week holiday was cancelled. 

His passport was then found under a hedge, as far back as it could have been, in a place already checked. 

I have looked at every possible person it could have been but there were only 2 other very slim possibilities - 4yr old grandson but he wouldn't have been able to reach to put it where it was and wasn't alone anyway or my 20yr old daughter who doesn't live with me but had been at my house and had no reason to ruin our holiday.  

I know in my heart and head it was my son but he's categorically denying it and gets angry that I don't believe him. I don't honestly know what to do because I feel he needs to acknowledge his actions and the consequences of them.