Thanks. I will look for those. I remembered just now that the colt became attached to the trainer who worked with him as a yearling–when the guy hauled him to my place to drop him off, as the trailer was leaving the colt actually ran along the fence next to the driveway and screamed for the trainer (but then did settle down shortly thereafter, with the two mares).
So, this horse has a propensity to become emotionally attached…but might “get over it” fairly readily too…it might be that he just basically wants to pitch a fit, then give up if it isn’t working. That’s why I think that eventually tying him to a patience pole might be good for him.
Clearly this horse doesn’t at present have confidence in me to make him feel secure in the absence of other horses. Unsurprising considering that I unavoidably fumbled around somewhat handling him as a foal–no big harm done, but he always had a mind of his own and my inexperience didn’t help. Also, I have backed off at times rather than get too forceful in the wrong way out of fear of “screwing him up.”
This experience has shown how little I knew about handling this type of situation. I’ve ridden a lot of green horses, re-started OTTBs, and even backed a couple but they were all at boarding or training barns with lots of horses around.
It’s a little surprising to me that this horse turned out to be insecure in this respect because he is actually pretty bold and unfazed by things in general–but as I found out, as long as his mommy is nearby. 
Re some of the other good points raised above:
His testicles were both well descended when he was gelded at 10 months of age. No problems there and he is not at all sexual with the mares–just emotionally attached.
That’s a great idea to make a round pen instead of a square or rectangle. I’ll put up a 30’ diameter pen which could be enlarged and moved into the arena if I can get the local clinician to come over and he suggests doing so (the arena will have better footing).
I am definitely sending this horse out (off the property) to be started. The guy who worked with him as a yearling is good and he will do it but he’s too far away to help with the current situation. As PaddockWood noted it’s risky sending them out to someone you don’t know enough about which is why I want someone to come to my place to help with the herdbound issue. I did get a good pair of gloves and a long lead rope (more like a longe line) to work on the handling when he acts up.
Fanfayre, one of the mares I currently have is actually very dominant and puts the colt in his place, so he behaves well around them. It’s me he doesn’t respect.
Re the suggestion to get another horse: actually, I have had my eye on a filly that’s for sale so appreciate the enabling. This is not a good time for me to be getting another horse, but it would help to be able to have 2 groups and mix things up. There is plenty of room and it wouldn’t be difficult to make it work property-wise…hmmm…
I think that with help including information from videos such as those from Schiller that this horse will improve a great deal.