While I can understand why you might think internal parasites might be his problem, it's not likely for an indoor cat.
When one of my patients starts to lose weight in the face of a ravenous appetite, then the first condition I'd want to rule out would be Hyperthyroid Disease. Some of these patients will have other signs such as the vomiting I asked you about but others will only lose weight while acting hungry all the time.
This is a relatively easy diagnosis to make with routine bloodwork: the T4 is usually elevated in these patients.
And, there are a number of very good treatment options which range from daily medication (which actually can even be compounded into a gel that is rubbed into the ear) to diet (Y/D although most cats don't like it very much) to radioactive iodine treatments (which can be somewhat pricey). I actually consider this to be a good diagnosis since the majority of patients can live quite a few years after treatment is started.
I'll usually also suggest that blood pressures be measured since Hypertension can be a secondary problem for some of these patients; the procedure can can be done at your vet's office if they have the equipment.
If Maxie were only losing weight but had a decreased appetite or other issues, then I might want to include other conditions on the list such as kidney problems or liver issues; however, Hyperthyroid Disease is going to be just about the only one which will cause a cat's appetite to increase so I'd want to test for this first.
I hope this helps. Deb