Still sucking

Okay, I'm too soft for this weaning thing. He's still nursing but there have been a few changes.
I was pleased to have Tatti to comfort him through his gelding, so that was good. Since then he's had a one-day fever (103 degrees) that passed quickly and a brief colic that also passed quickly. If he had been weaned I would have blamed it on that, but there didn't seem to be any obvious cause for either. Kids stuff. 
So he's still nursing. I was trying the fenceline weaning and separating them for longer intervals until it dawned on me that they only milk dairy cows twice a day so how was that different here? Then the nutritionist mentioned that a foal couldn't deal with forages at full capacity until they are 8-9 months old (I would like to confirm that so don't quote me yet) but I've taken that to explain why weanlings always look a bit potty. Jake doesn't.
I have worked with foals who were weaned at 3 months old and while it is commonly said that the milk is just washing down the food at that point, I don't know how true that is. I do know that they are not mentally mature enough to deal with it. They need their mothers for longer than three months.
The typical weaning time is 4-6 months but I wonder if that is optimal for the foal? It is convenient for us but what is best? In the wild does the mare nurse her foal until the next foal comes along? Since it's winter I'm going to be doing a bit of reading. I'd love to hear from you if you have any information about weaning in the wild.
Meanwhile, I've increased Tatti & Jake to twice a day grain (Buckeye Gro'n Win and crimped oats) and both are holding up well. Jake's now 13H and 520 lbs., so he's about half his mature weight at 7 months and about 85% of his mature height of approximately 15.1H.  Tatti's weight is good and so is her sense of humour, so I'm going to carry on and procrastinate about this weaning thing for a while. 
Last week we went out for a trail ride - I pony Jake off Tatti when I feel like a fun adventure in the woods - and when he got back he went to the water trough for a drink instead of the boob, so maybe this will just take its course naturally.