Hi,
My gorgeous DD has Down's and has just turned 3. Whilst sensory websites are great, they do tend to be expensive, and at your grandsons age you should be able to find similar things in any toy shop.
What type of thing does your grandson like to play with?
At that age my DD was obsessed with musical and/or flashing toys, and absolutely loved this toy , in fact she still plays with it. We ended up with 2 of them, so that we could play along with her which was great fun.
She also loved anything with a mirror on it, they sell some great ones at Boots that have lots of flaps, ribbons etc. to explore.
You mentioned your grandson is almost sitting, does he have a bumbo? If not, you may want to look into getting one of those (about £25 available at boots, mothercare, john lewis, supermarkets etc.) as it supports the child in a sitting position. My DD had one & it was great, we used to sit her in it with the tray on & she could play with all her toys with both hands. It was also really useful for feeding, as she struggled initially with a high chair until she could sit independently.
HTH & happy birthday to your grandson!