What a lucky boy, and what a superstar you've been to take responsibility for him
Agreed he sounds like he's struggling with pain, or possibly the memory of pain. I would ask the vet about some metacam for him, and dose when he seems particularly down. Like Gill said, it can make a real difference but you need your vet's agreement as it isn't suitable for all cats.
There's another thread running here with a similar topic, you may be interested to read through
http://www.purrsinourhearts.co.uk/index.php/topic,41919.0.htmlCats in pain will associate places and people with that pain. e.g. our Theo was taken in and cared for wonderfully by a local cat rescue manager Anna, after being hit by a car - but after she re-homed him here, whenever Anna visits us, Theo gets very angry and stressed, and has feigned attacks towards her before now.
I dose Theo metacam when his behaviour turns angry or solitary, as his pain comes and goes in waves, and is reflected in his behaviour. He needs it probably only one or two days each month. My vet is happy with this, and accepts that giving Theo a full dose occasionally is doing no harm and Theo is much happier.
I wonder if your boy is associating his pain with being handled by you, which is why he's showing defensive traits.
Anna advised that we have to be guided by Theo, if he doesn't want to be handled, then don't do it. Just being consistent with a supply of treats, food, a little play with laser and valerian toys and he lets us know when he's ready to be loving again.
If you can get some pain relief into him, you might find a completely different cat underneath, who may well turn out to be much nicer than the poor sad boy you're currently seeing.
I also note he was recently neutered, so he will also be adapting to unfamiliar feelings due to the hormone changes this brings about. This may too have contributed to his loss of confidence, and be another reason why he's reluctant to venture out.
As for his weight, do start hiding the night food away, your other cat will adjust quickly. Then a diet made for indoor/less active cats will benefit his weight control. Most of my lot eat James Wellbeloved senior Turkey and Rice which achieves good coat condition and weight control whilst having no grain - so my grain intolerant boy can eat with the others.
Polly on the other hand prefers Purina Indoor formula, which I find is good for her regularity and weight control.
Catnip toys might get his attention, or he may prefer valerian. This is the one my outdoor boys go particularly crazy for, and may work for your boy.
http://www.purrsinourhearts.co.uk/shop/best-cat-toys/Cat-toys-valerian/purrsWiffeezz-yettiez-valerian-cat-toyStress wise, I'd start with a little Rescue Remedy in his drinking water, and maybe sprinkle a Zylkene on his food for a few days to get him relaxed.
http://www.animeddirect.co.uk/zylkene-capsules-priced-per-tablet-75mg.htmlOnce he starts to trust you and the pain is under control, the grumbling should reduce and he'll eventually come to you for attention - especially if you use this...
my secret weapon (actually not so secret
) I never fail to be amazed at how much they love this, and it's so good at building trust and loving them better