Mklh - how's it going w/ your cat? It's not at all unusual for stray cats to take a long time (months or more) to warm up, especially since they're changing from a predominantly outdoor environment to an indoor one. It takes alot of patience to take on a feral cat & I certainly applaud you for your efforts. A couple of things you can try:
1/ Stimulation for play - the cat may be lacking stimulation for play situations. Try mice, balls, rope...maybe dip the mice in cat-nip. See if you can get the cat to play & perhaps tire out a little, especially before bed-time. That will help him relax & also give some more interaction between you and him. Other play ideas:
--> A light-pen...alot of cats love to chase a laser pointer.
--> "Treat" games - for example take treats and put them inside a cylider (e.g. a used toilet roll) with some holes for the treats to come out of. The cat can spend hours pushing round the cylinder to get the treats out.
2/ Vet & possible medication - try talking to your vet. If you need a recommendation 9 lives (
http://www.9lives.com.hk/) is a great vet & might be able to diagnose if there's something specific. Also there are certain medications you can give to relax the cat, some very mild (like chamomile drops, put in the food/water) and other more powerful (like anti-depressants). See if your vet has some ideas.
3/ Environment - see if you can add some "outdoor" environment indoors. Maybe a large plant with a box or hiding space for the cat nearby. Little pot-plants with herbs can also help (they're aromatic & provide that outdoors smell).
4/ Pairing - this may or may not work, but is a worth a try if you're out of options. If your cat was used to being with other cats, one way to improve his outlook may be another cat. You could try fostering from a shelter and see if this has any effect. It's not always easy to find the right match. Some cats "take" to each other right away and others refuse company.
This ended up being a long post! Do keep us updated on how it's going.