Jasminum nudiflorum ... pretty yellow flowers.. mentioned on a recent question by Anitalynn about climbers.. This "Winter Jasmine" doesn't exactly climb, but will reach maybe 6 ft. if tied against a trellis... evergreen, and will flower every winter, even when the weather is cold and icy ..

Sorry to learn about the death of your son's cat..:o(

I have a winter jasmine which I bought at the beginning of the year,and the bright yellow flowers certainly add a bit of colour.

They are small flowers, but so eye catching. Why not google winter jasmine? I think you might like it.

Unless the cat has been buried close to a fence then the jasmine might be a bit difficult to fit in. Winter flowering heather would be the easy solution as you can get them to flowerin all months. Someone might have a cleverer idea.

Erica carnea, as suggested by Bulbaholic - for something a bit larger, Chimonanthus praecox, commonly known as Wintersweet. Hamamelis mollis can also be flowering in December, but usually a bit later.

It isn't made clear whether the requirement is a flower to go anywhere in the garden, to remember the cat, or if it is to be planted over the grave ...

Look in the garden centres for hellebores (helleborus x hybridus), some of which flower earlier than their type, and an early flowering plant tends to keep the habit. Mahonia x media 'Wintersun' is an option, if you don't mind the slighty prickly foliage (to remind you of the cat's claws!), and possibly camellia sasanqua 'Crimson King', although that tends to be a bit later. Got room for a small tree? Prunus subhirtella 'Autumnalis' (small white flowered cherry), gently trickles on from late November through 'til early spring, and the cultivar P. subhirtella 'Autumnalis Rosea' (pale pink), is very pretty, too. Worthy

Lonicera fragrantissima or L. Winter Beauty or L. Budapest all flower in a normal December and are sweetly scented, in order, yellow, white and pink.

I was also going to suggest a Helleborus. I dont know how old your son is but one idea I have is your son could choose a large pebble and paint the name of his pet on it before clear varnishing (3 coats) it to put with the plant.

Thanks for all you replies, very helpful indeed.
Hard to know which to go for but I may go for a heather (Calluna vulgaris) as Calluna is dervied from greek to mean 'to beautify' or 'to sweep' (as it was used to brush with) and the cats original name was ....Sweep!
He was known to us as Smudge.
My son and I love the idea of varnishing the stone.
Thanks for all your replies, very kind of you, and I shall let my son decide which one to plant.

Bear in mind that Calluna must have acid soil,will not tolerate any alkalinity at all - Erica, the other form of heather, is much more forgiving - prefers acid, but will tolerate some alkalinity. Of the two plants, Erica carnea is more likely to be in flower on 8th December than Calluna, but there's no guarantee with either - depends on the mildness of the weather and its situation.

Perhaps some spring flowering bulbs too?

I've just thought of something else - Neriums - mine are usually still in flower at the beginning of December, though this year, the flowerheads have been knocked by being frozen last week, although they're still present.