I'm so sorry Draco has been vomiting shortly after eating. This could be caused by various factors. He could be eating too quickly and with dry food, the way to prevent this, is to place a new, washed golf ball in the middle of his bowl to slow him down.
Another reason could be excess acid in his stomach or acid reflux, which causes nausea and then, vomiting. To prevent this, you can give him 1/4 of a 10mg. Pepcid AC (Famotidine is the generic) tablet, about 30 minutes before his breakfast, and then again, 12 hours later. Here is more information :
I'm glad you stopped him from going into the garden, as eating grass and any herbs can cause him to vomit, and you don't know if any herbs or other things around your garden, like plants or flowers could have been nibbled on and are toxic to cats. If you saw any grass when he regurgitated (it's really regurgitation, not vomiting, when it's shortly after eating and contains undigested food), then that would give you a good idea of the cause.
Wet food is really healthiest for cats, but you can leave a good quality dry food around, in a portioned size, only at night or during the day, too, for 'grazing', which cats love to do. The best foods have real meat listed as the first ingredient and don't contain any fillers/grains like corn, soy, wheat, gluten or carrageenan, artificial flavors, colors or preservatives. This is especially important for older cats, like Draco. I think you'll find this article written by a feline vet, interesting: http://catinfo.org/
For the moment, try the Pepcid AC to settle his stomach and eradicate any nausea, monitor him for any further vomiting/regurgitation, make sure he's urinating and defecating as usual and don't allow him to eat any more grasses, as you mentioned.
If you notice any more unusual symptoms or a continuation of the vomiting, have him seen by his vet in person for an evaluation and a senior blood panel, including a thyroid test. This will provide additional diagnostic tools for the vet to make a diagnosis and prescribe treatment, if needed.
I hope all will be well with Draco!
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