Mine is 10yo too, with ASD and other co-morbidities, and although he is verbal, he has cognitive impairment and learning disabilities, so there is some commonality between your son and mine.
Mine absolutely loves water. So on a warm day, he would love any of the following
To fill a watering can with water from a paddling pool and pour it anywhere and everywhere including on himself, but he likes to water the plants and the oaths too. And then he likes to watch the water drain away down an income or steps or whatever to see the oaths and patterns the water makes. He will watch this and use his fingers to make patterns with the water
As above, but will fill up the can or a jig from an outdoor tap (this obviously needs more supervision!)
Play with the water hose, using it to water the grass or fence or himself or spray other people (maybe one for just before bath time!)
Fill up water squeezers (bath toys) and spray them on the ground or on his toys
Wash his toy trains and cars in a bowl of water and make them nice and clean (it’s all about the process, not the result!)
Marks or drawings with chalks on paving stones or pavement
Painting - we have these big cardboard kits from hobbycraft and just let him loose on them in the garden
Painting again - he likes to paint himself, so again I just let him do that either with face paints and a brush or wash off paint and a brush and let him sit in his pants and he will paint himself green usually, to look like a frog
We also have a load of these paint your own sets from hobbycraft
He also likes building dens, using blankets and chairs and things like that
He also likes gathering things. So gathering stones or sticks from our (messy) garden and piling them up in a shady spot, which he used to liken to makka pakka’s stone house, or a three little pigs house
I’m conscious that lots of these are outdoor idea and I don’t know if you have outdoor space. If you don’t, some of the painting type things can be done on the kitchen floor or in the bath?
The bath is well used in this house for play, even in the day. We have lots of bath foam, the crackly things that you can put in the bath, bath bombs etc as well as squirters and pots and jugs and stuff. He takes lots of things into the bath like duplo, some plastics trains, balls etc.