We have a story that is very similar. One day when emptying some lawn clippings into our waste bin, I saw a fuzzy white lump in our recycling bin. It was a 2-3 week old kitten, all by himself. Here’s a pic showing his actual size compared to my hand:
http://public.fotki.com/mzeddean/oliver/oliver/image018.html
We took him to the vet that morning and everything looked fine, so we bought a feed bottle and powder to start hand rearing him.
I seemed to recall a life lesson that is a stray dog, cat or similar turns up on your doorstep you should take them in. It’s a karmic thing, I think.
Later that day his mother showed up, and we realised then that his mum’s milk was best for him. It was a karma thing: if he was meant to be with us he would come back.
Three weeks later, he did. He just showed up on our driveway!
http://public.fotki.com/mzeddean/oliver/oliver/oliverkitten.html
He waltzed in and, after a few token hisses started to make himself at home. We took him back to the vet: he was a little undernourished, and crawling with fleas but aside from that he was in good shape. He was well into solid foods and figured out what a litter tray was for in very short order.
And, 5 months later, this is oliver today. Beautiful nature, very much a member of the household.
http://public.fotki.com/mzeddean/oliver/oliver/dscf2096resize.html
The point of my story is, yes, definitely take him to the vet and immunised ASAP. Based on the size in the photo it may be a bit too early for solids, even crushed down. Anyway I think kittens need alot of liquid in their early diet as they are at risk of dehydration. Your vet will give you good advice here, I am sure.
good luck! And my tip for a name is Hera: it’s had a second chance at life hasn’t it?
gaf