bowel problems - other dietary/fat considerations?

I am surprised that you weaned him at 6 months, I think you should have been still breastfeeding him up to 1 year (at least) unless you have circumstances that truly make that impossible. But I guess it's too late now.

Remember that babies need way more fat (percentage-wise) than adults. Especially essential fatty acids. That includes omega-3 and omega-6 in the proper ratio (something like 1:1).

I noticed you mentioned flax seed oil, avocado, coconut, etc. Now, I am all for vegetarianism and vegans, but (don't shoot me here) I have read that a LOT of vegan babies have problems. It is apparently too strict for children up to 6 years of age or so, no matter what anyone says. I have also heard that flax oil has potential physiological problems.Now, I definitely am not suggesting that you start feeding your child steak, or dairy products.
Might you consider cod liver oil, or fish oil?
See this link about a top quality fish oil: http://www.mercola.com/forms/carlsons.htm

Dr. Mercola states there:
While a helpful form of Omega-3 can be found in flaxseed, walnuts and a few other foods, the most beneficial form of Omega-3 - containing two fatty acids, DHA and EPA, that are essential to fighting and preventing both physical and mental disease - can only be found in fish. ......
And so I recommend, above almost any other addition to everyone's diet -- from infant to senior and every age in between -- fish oil and cod liver oil.

Here is an excellent article by a different doctor about Omega-3, Omega-6, and why Flax seed is not the best supplement for Omega-3:
http://www.mercola.com/2002/apr/3/evolution.htm

Of course, I don't know that this is the reason your child is so constipated, but thought it would be good to mention anyway. It is very interesting that he became so constipated upon being weaned. That is terrible for anyone to be impacted, much less a baby, what will his future be like?
I think you should continue with enemas at least twice weekly for a while, keep up the probiotics, and massage his tummy clockwise, take him to a chiropractor, etc. but you really might want to consider the issue of adding fish oil, and maybe half-cooked organic eggs to the child's diet for a few years. Hope he improves!
-Squeaky Tweet