My 7 year old son is throwing things off kitchen bench

Good morning all, my name is Vicki and my son is called Alfie, he is 7 years old and has autism.  His speech is very limited and he uses PECS.  Through the years we have experienced lots if different behavioural issues such as throwing objects out of Windows, tipping liquids down the sink, tipping drinks over on purpose, slamming doors persistently, hitting, kicking.  We are now experiencing probably the most challenging yet, Alfie is throwing objects off the kitchen bench, off my desk, throwing of window sills, dressing table etc.  We have lost two microwaves and several pictures and trinkets.  When he does it he is laughing his head off.  We initially thought the behaviour was for attention, but now believe it to be a game.  We also thought it was to do with me and my husband, when he's away he doesn't do it then when he returns he does??? This theory got blown out of the water yesterday as while my husband and I had the rare opportunity to go to a football match together, Alfie took it upon himself 2 hours into being babysat, that he was going to tip throw things off every possible surface in the house! In the end my babysitter took Alfie out to her house to stop the behaviour escalating even more.  Once at hers he was an absolute angel.  I'm really at a loss with this one and am looking for any advice or ideas from any of you fellow parents/carers...help x