My cat is dry heaving, drinking water but not eating,

Okay, thanks for this.
Her presenting complaint is quite non specific as we can see a lack of appetite and dry heaving in a number of conditions. It can be linked to nausea, which can be seen in kidney disease, an overactive thyroid , certain cancers etc.
It may also be a more local issue, for example an upper respiratory tract infection or a foreign body.
Given her and age and history of possible weight loss, I would really want her checked over from nose to tail. As well as an all over exam, a general blood test would likely be indicated.
Best case scenario, she just needs some anti-inflammatories to settle down a laryngitis but we need to rule out anything more sinister.
We also want to ensure she does not become dehydrated/weak from the lack of food. Sometimes, these kitties will need syringe or tube feeding until better.

Given the fact they are associated with lying down we would also really need to assess his heart and lungs for any signs of murmurs/crackles etc
At home, you can count his resting breathing rate which should be less than 30 breaths a minute when resting or sleeping. Any more thab this indicates an issue with his oxygenation

This is a concern. Whether he has something stuck in his throat, there is a severe inflammation or there is an underlying issue with the heart or lungs, we would really want him checked out asap.
If you can, it would be best to contact your vet now to have him assessed. Cats can be masters at hiding any signs of being ill so behaviour like you have just mentioned is a red flag.

Good evening, this is just Dr Linda checking in to see how Scampers is doing?