Please remember I am not a SALT. Our techniques were very different from the SALTs I saw when they visited 'our' kids. Our techniques worked pretty well with autism, but I have no idea how well they would work with any other children. Also, I have no idea of what sort of stage your children are at, nor what age, nor how they learn. Everything we did was geared towards each individual child and what worked best for them, but the technique was always pretty much the same. Teach one thing, then a second, then differentiation between the two, then introduce a third (by itself), then mix it with the first two etc etc etc. Small steps all the time and lots of reward for getting it right.
But, FWIW:
More categories yes: cutlery, crockery, rooms, buildings, toys, anything you see around you on a regular basis. You can use photos, models or the real things. We had to teach the skill of recognising that a picture of a ball is a ball iyswim, so we had lots of intermediate stages before we could really start on categories.
You can use this technique with pretty well anything you're trying to teach. You can then ask for things like red squares and blue triangles, for example, with some eg. yellow squares & green triangles mixed in.
You had food, transport, animals and people. If there is some way that you can mix these items across the major categories, wooden, green, big (not so easy as it requires comparison) to show that things can be categorised in different ways, big apples, red apples, shiny apples, half-eaten apples etc.
You can extend your requests: can you find the blue wooden ones, the big cars and so on. Use things which are familiar to him, like cutlery or crockery.
You can make your requests different too (this would a whole other ball game when we were working with autistic non-verbal children) so instead of asking for things to go in the basket, you could suggest he puts the animals in the corner, or on the bed etc.
To improve receptive language, you need to extend your 'requests', so you would be saying (eventually!) please put the big plates in the cupboard or please give everyone a knife and fork (at dinner time). Then you need to start using different words, for instance, stack the plates in the cupboard on the left.
Get things into the real world so it becomes truly meaningful.