Awww, poor thing. I know it must be heart-breaking to see him miss his sister like that.
I think this is the kind of heartbeat toy CatGirl was mentioning which may help for when you are at work?
http://www.ebay.co.uk/itm/like/150689447753?hlpht=true&ops=true&viphx=1&lpid=95&device=c&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0&ff12=67&ff13=80&ff14=95
We have lost 2 cats over the last couple of years due to old age (24 and 23). When the first one passed away we also had a 2 year old kitten, but also got a 5 month old rescue kitten as it didn't 'feel' right with just the two. The 24 year old didn't really mind the kitten at all and just carried on doing her own thing. I think this is because the kitten had the 2 yr old cat to play with. So perhaps if you are thinking of getting another cat, two young ones together might be a good idea. This way your older cat will have feline company, but the younger ones will run off some steam together. Providing the older cat has a quiet area somewhere for a bit of alone time when needs be.
I understand it may be tough to decide whether or not to get another pet after losing one you loved, and I hope your boy cat starts to get back to himself again soon whatever you decide.
Ps The feliway plug ins are a marvel, they work brilliantly.
All the best