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Hi this is Dr. Sara
So, most likely it is a combination of the dewormer, new food and stress of a new home. However, a kitten that small will dehydrate quickly and this may be why he is not pooping. Do you already have a vet appt set up for him?
If he is still eating and drinking/peeing and pooping you should be alright. If your vet is open today you may want to schedule an appt for tomorrow am just in case. If he's great tomorrow you can wait until your appt next week, if he's still vomiting you can bring him in for an exam and fluids.... It always seems that things get worse on the weekend when nobody is open except emergenciy hospitals and that can get expensive.
Please let me know if you need anything else I will be checking in tonight.