I'm sorry to hear that Rambo is not feeling well. It's possible that the new food is not agreeing with him and if he had any chance to lick the flea treatment, this could be related.
What dry food did you change to and what brand of flea treatment did you apply? If you didn't get the flea treatment from your vet, there are many brands in supermarkets and pet stores that can cause very adverse reactions in cats; some even fatal! They sell Advantage and Frontline in pet stores, now, so that's okay, but if you applied Hartz, Zodiac, Sergeant's or another one from the pet store or supermarket, this could be Rambo's adverse reaction, even if he didn't lick it, but just from the application and being absorbed by his skin. If you did use a brand not sold by your vet and it's only been about a day, try washing it off at the site of application with warm water and original Dawn dishwashing liquid or any dishwashing liquid you have in the house (containing NO bleach, crystals, etc.--just plain). Rinse well with clear water.
If he's vomiting, for the moment, don't offer any food and for kittens and cats in general, high quality canned food is best as their main source of food, but you can keep a measured amount of high quality kitten dry food out during the day and at night, as cats love to 'graze.' Good quality canned and dry kitten foods will be grain-free, real meat (not by-products) will be the first ingredient listed and it will contain no artificial anything.
Is he urinating as usual or not at all? Male cats are more prone to develop urethral obstructions, which can be life-threatening and vomiting is one of the symptoms. If he is not defecating, he could have an intestinal blockage and if his waste is backing up into his body, this is causing him to become septic and he's vomiting and lethargic, due to that.
For the moment, you can give him 1/4 of a 10mg. tablet of the over the counter acid controller, Pepcid AC (generic=Famotidine), and this will help soothe his stomach and help with the vomiting. An empty stomach produces excess acid, this, in turn, causes nausea, so that's why this medication may help.
Also, rub some honey or Karo syrup onto his gums, to give him some sugar, as he's probably hypoglycemic and this might perk him up.
However, because of his young age and sudden onset of inappetance and vomiting, it would be best to have him evaluated in person by a vet, as soon as possible. He may be dehydrated from losing all that fluid in the vomiting. The vet can give him I.V. or sub-Q (under the skin) fluids, to help with that. You will get a diagnosis and he will be treated.
I hope all will be well with Rambo!
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