Don't introduce a new cat to the environment just yet. That's the common
wisdom, anyway, at least from what I've read. Wait a bit, then try
introducing a new feline into the household, once he seems to start
accepting the irrevocable absence of his friend. However, don't expect
that your pet will readily take to a new cat. He might become so used to
his only-cat status by that time that he'd be resentful and treat a new
cat as an intruder. But it's something to think about down the road,
maybe in a month or so.

Also, don't let a vet talk you into euthanizing your remaining cat. That
to me is just sheerly heartless. Your vet is letting his/her inoculation
needle do all the thinking.

Cats will mourn the loss of any household companion for up to six
months. Although in cats, it's not properly mourning; as hunters, cats
understand death and so don't grieve. The more accurate term is
abandonment depression. Your cat's years-long companion is suddenly
missing, and he's doing exactly what cats do in this situation. I've
seen it in my own cats over the years.

Right now, your poor guy needs special attention. Don't force it on him,
though. Just give him a little extra affection from time to
time--treats, a new toy, some extra petting and cuddling. This would
also be a good thing for you as well, since you're in a state of grief
and need whatever comfort you can get from your remaining beloved Siamese.

But for now, he's doing what a cat does when it's suffering from
abandonment depression. If it goes on for too long, you might ask your
vet for an anti-anxiety med for him.

More good info is available at http://www.messybeast.com/cat-grief.htm.

HTH.