Does my kitty need a therapist?!

My neutered/vaccinated male, Sandy, has always been a nervous cat, but he's progressively gotten worse over the past few months. His story: Sandy, and his brother and sister, came to our yard with their Mama as ferals almost a year ago (he is now approx. 13 mos. old). They were all so wild, but all three kittens fianlly tamed to me - Sandy was the first! I can easily pet and hold him without problem - he enjoys being held and cuddled. He has always been a bit of an isolationist, but he's gotten much worse lately. He loves his brother and sister, but it seems he would prefer to be an only child. Today, it was the worst it has ever been. He must be an outdoor cat as a family member has severe asthma. We live on an acre next to open fields with many dangers (coyotes/tractors/etc). This morning when we all were taking our morning walk, over the 6 ft. chain link fence Sandy went! I tried to remain calm and call to him, but he would not come to me for anything! The other cats stayed right next to me and seemed frightened. He finally did return to his yard when tractors started working in the field. I did get to pet him once he returned, but only for a brief moment - he smelled my hand and ran off! PLEASE HELP!

My questions:
* Has anyone had experience with a nervous cat like this? What has helped?
* Did neutering him make this probelm worse? Why is he wandering after being neutered? He was neutered at about 8 mos. - is he mad at me for that?
* How can I stop him from jumping the fence? Should I use a shock wire on it? Barbed wire?
* His Mama is no longer with us because she was diagnosed FIV. Sandy's vet said he didn't have it, but I don't know if the actual test was performed. As you can imagine, Sandy is not the most willing of vet patients. The staff checks him out/vaccinates him as quickly as possible, and he's gone. Could he be sick? His coat is in good condition (silky/soft/shiny); he's a little thin, but so are his siblings - they're all very active!

Any help/adivce is GREATLY APPRECIATED!!!!!

THANK YOU ALL SO MUCH!!

If you can afford it, or if someone there is handy (and it sounds possible from where you live if you know what I mean), a good compromise could be an enclosure, plus daily harness & leash walks. He'd get to be out and eat bugs, but stay safer and under your control. Check out various types of enclosures for safety features, etc., and see if it's possible to put one together as large as you can manage. He's not mad at you re being neutered - probably had no memory of it beyond a week and certainly no understanding of what it was about. He's just a young, active cat who probably smells other animals or birds 'out there' and wants to explore.