As far as the testing goes every one I have ever heard of was simply directed playtime with the skilled observer offering toys and activities that will tell them what they are looking for. The only advice I would offer is to sell it as a fun playdate and to have as little anxiety as possible so that he is as open as possible.
If he does have autism then maybe my explanations of his behavior will be helpful, if not then they mean something else.
It sounds to me as if he has a hyposensitve propicieptive(sp?) sense. A mouthful-huh? In layman's terms:
We actually have 7 senses most people are not aware of the last two because they just work and we don't notice them but autism can cause them to be either hyper(too much) or hypo(too little) sensitive. The vestibular(sp?) sense which is our awareness of our body in space it's the sense that tells you that you are right side up and facing forward it has to do with balance and willingness to jump of a low platform. The propicieptive sense is the awareness of pressure on our joints. When you press on a door it compresses the joints all the way from your wrist to your spine and feeds you information about how hard to push until you feel a lessening of the compression and that tells you you have begun to open the door.
If a child with autism has a hypersensitive sense they will avoid the stimulus if they have a hyposensitive sense they will often do behaviors that will give them stimulation of that sense. Many children(mine included) will enjoy pushing on heavy objects to provide them with strong joint compression they are looking for. They make weighted vests and blankets that provide this stimulation but there is debate as to the safety of them. I find hard bear hugs(given often) are better and cheaper. I have also heard the suggestion of getting them an umbrella stroller or tonka truck loaded with weights(like sandbags) and allowing them to push that around. My son likes to take his stroller and push it through deep sand. He also pushes us but he can be distracted with his stroller often against a wall or loaded with heavy toys.
is either hyper or hypo sensitive pain response. Helpful huh? Seriously, some children do not like to wear clothes because the are hypersensitive and the feel of clothes irritates them. Or he is hyposensitive and the barefoot, cold, painful escapes are seeking stimulation. You can probably narrow it down by noticing the circumstances it usually occurs under.
The drinking of water(if all medical conditions have been ruled out) may be a sensory thing. Many autistic children like to play in and pour water perhaps he likes the feel of it in his mouth. If so giving him ice may slow him down and satisfy him without so much intake. Unfortunately his grinding of teeth is probably more of that prop. sense so it may lead him to crunch the ice to feel the compression of the jaws(it should still slow him down somewhat and may lessen the teeth grinding). My son crunches hard candies, bites lollipops right off the stick, and crunches ice. Many children with autism have to have corrective dental work for teeth grinding.
Finally if his diagnoses is autism I would get a book called ten things every child with autism wishes you knew. It is a short book and written by a journalist mother of an autistic child. It is easy to read and explains much of what i have touched on here(except much better). It is a fabulous starting point to understanding what i know seems like a very confusing subject. Good Luck
p.s. http://18.104.22.168/search_site/chapter_map.cfm This is a link to the autism society of america chapter locator. Find the chapter nearest you and contact them they will be your best freinds