taraann, weaning by weight is fine, although I personally would wait until 23lbs rather than 20. I've done 20lbs before but in my experience it usually only takes them another week to hit 23, and that extra week does them a lot of good. But only wean if he is eating his grain well.
Your replacer looks pretty good ... the key bit there is "Protein from Milk Sources" 22%, total protein 22% so it is a whole milk product, which is spot on. I tend to use something a touch higher, around the 25:25 mark, but from what I understand about American milk replacer products you dont get as high quality as we can get here. And as long as its over 20:20 you are doing well.
Reconstituted Kid-Gro may be refrigerated up to 24 hours. Discard any unused portion after 24 hours. If warmed in microwave only use medium setting and mix thoroughly and regularly. Check temperature before feeding.
Make sure you are doing this ...
About the castor oil I'm honestly not sure. I've never used it, I know you can give them cod liver oil though.
If I had to use a US replacer this is the one I'd use
http://www.mannapro.com/nurseall.htm
But if your little guy has been going alright on the other one, I'd talk to your vet, its probably going to cause more harm than good switching him over now ... if you do decide to switch him, do it slowly by mixing it in 1/4 to 3/4 of the normal stuff, for a few days, then 50:50, then 3/4 new stuff, 1/4 old stuff ... you get the idea.
Its good that he'd had a movement since the oil ... if you think he needs it, you can give the oil daily for several days. If he's moving some and still happy, eating and playing, I wouldnt do the enema yet, only if he goes down hill