we always get hard dark small poop about 3 times a day
Personally I'd want to sort this out as priority. If it was me I'd cut back on most/all solid food and reintroduce 1 or 2 at a time and see what happens. I'd probably keep start again with fruits and prunes to get the bowls moving more smoothly. If there is a blockage you may need to see the doc again.
Here's a helpful post worth a read:
Constipation and Laxative FoodsI discovered my DS's poo got much firmer with carrots, if I gave him too many he would have ended up constipated but once I was aware of it I was able to balance his diet giving some prune to soften and carrot to firm up.
I wouldn't worry about any other aspect of solid until this is resolved.
how to give him some proteins
LOs can suck and gum on large hunk of meat rather than ground down or pureed meat, maybe try that, a large piece the size of an adult finger. This should be big enough to hold in a fist grip and still have a bit poking out the top which LO can gum on.
Otherwise there are many other sources of protein. Fish, beans, lentils, eggs...
Mine was not a fan of meat and still isn't really. He does like a very slow cooked piece of meat (in stew or in the slow cooker) so it is super soft but prefers fish, beans and eggs. Maybe some home made falafel to offer as finger food?
finger food.
Opening the hand to release/drop an object and passing hand to hand is a developmental area which it sounds like your LO hasn't gone through yet. it will come. Meanwhile size all finger foods large, adult finger size or chunky wedges, big enough to hold in a fist grasp with the bit poking out of the top. Help your LO hold it in such a way as to be successful. If he needs it handing to him that's fine, he will eventually be able to pick it up himself.
For wet fruits try a plastic fork, stab the food for him and let him hold the fork and bring the food to his mouth, guide a little if needed.
Often you can cut the fruits in a different way to help with grip too. For melon try cutting in a crescent shape rather than long wedges, the curve of the crescent gives an easier place to grip. it can even help to misshape wedges/fingers/chunky cubes by just squishing one side with your finger, again it gives a kind of finger hold so the food slips less.
Some fruits you can bake in large wedges in the oven this makes the outside dry whilst the inside is very soft (eg apple, pear, also veg such as sweet potato).
Puffs and cherrios are likely just too small for your LO at this time. He needs a pincer grasp for those little things and this is another developmental area. For cereal finger food try things like fingers of toast, pitta, naan, chapati, oaty chews, porridge fingers or rice cakes, things large enough for him to hold and manipulate.
I really wouldn't be concerned with switching to a sippy cup for milk at this stage, he's doing well to use it for water. TBH the weight of a full milk feed in a cup is likely just too much for a LO to lift and control well. I'd continue with your regular milk feeds and enjoy the cuddle you both get during it. For water only put an oz or 2 in the cup so it is light enough to lift and control.
Hope this helps some