Play dough, but you will have to supervise and help him or he will eat it or squish it everywhere.
If you have an iPad, you can screenshot pictures of things to help him develop his vocab. I have one for my grandson (when I am able to spend time with him) with things like pasta, table, bath, cloud etc. He loves swiping the screen to move to the next pic. It's only a 10 minute task but he enjoys it.
Play horsies on the floor on your hands and knees with him on your back. My grandson thinks it's hilarous and will shout 'horse, horse' at me when he wants to play.
Duplo is great and your GS is just getting to be old enough for it. My GS has had it since he was 1. Again, you'll need to join in to help him but it won't take long until he has the hand of it.
Stickle bricks are fab and obviously, not small enough to be a choking hazard.
If you're allowed, playing in the bath with bubbles is fun.
A big blow up ball that you can roll backwards and forwards to him.
Balloons are fun. My GS loves batting them around and they are harmless if they bounce off things.
If it is raining, taking him out to splash in puddles in wellies is fun because it feels naughty.
Beaches are good, if you have a car seat and can drive. Particularly once the weather is not warm, because the beach will be deserted. Check the tide times and go when the tide is out and there will be rock pools to explore. I put a warm jacket on top and put him in trousers that can be rolled up. Take a complete change of clothes with you though, because no toddler can stay dry at the beach!
I envy you. I would love to be able to have two days a week with my grandson. The first lockdown was lovely for me because I wasn't at work and spent it all with my lovely boy.