Schema (sorry misspelt it) is basically a repetitive pattern of play, it is how children learn and if you look closely most children will do this to some extent especially when younger so it's not a sign of autism as such but it seems to be more obvious with neurodiverse children in my observations.
Your son sounds like he is a trajectory schema - interested in how objects move - often throwing items, exploring water etc. This is a schema often seen with babies and it's their way of seeing how things work.
If he is behind, be that speech, understanding, socially, I as a nursery worker would always say 100% to refer (especially as waiting lists can be so long) we have had a lot of children that haven't spoken at 2, I would say a large proportion are then speaking before starting school so it can take some children longer and speech does effect most other things in a child's development, it is a base of the pyramid so to speak, so he would seem behind in other areas. However if he is not understanding what you are asking of him sometimes and he's not engaging in play properly these are signs of something more I would say.
Also just because a child is autistic doesn't mean they will not achieve or will be held back, there are many functioning autistic people (myself included) who have done very well and early intervention is key to this! Get as much support as you can so that he catches up, one day it will all click and it will come together.
In regard to not knowing how to play this is why simple hand over hand actions are great, you are showing him what to do. I do this day in and day out with children who don't always know how to use the toys be that something simple like to pour the sand instead of throwing it or how to connect construction toys. So say a child threw the sand I would say why don't we pour the sand instead, I would then show them by pouring the sand by the spade into a cup for example. If they then went to throw the sand I would then take their hand to pick up the spade and help then scoop and pour the sand and then praise them, using simple language such as spade, scoop, pour as I did each stage. Once they get this simple step I would then perhaps introduce a water wheel to pour the sand into to make it spin. Again showing them and hand over hand if they needed it. Once they can say single words you would then encourage them to join words, blue cup, pour down, spin round etc.