Hi there - I'm Dr. Sara. I'm sorry to hear that you're having such a rough time with your kitty. I know that it can be scary and frustrating when they are sick and not responding to medications.
Lymphoma is a very common cancer diagnosis in kitties and it sounds like your vet has gone beyond what would be considered average treatment. Many vets aren't experienced with leukeran or other cancer medicines besides prednisone, so I applaud your vet for giving it a try.
Unfortunately even with those two medications, it's not uncommon at all for lymphoma to progress. The idea with treating cancers in pets is to maximize their quality of life for as long as possible without making them sick with the medicines. Two months is actually a fairly long time for many cats diagnosed with GI lymphoma.
It can be tricky at this point to determine whether the medicine (specifically the leukeran) is causing his blood cell counts to be low or whether it's a sign of his progressing disease. Lymphoma can spread to the bone marrow and cause abnormalities with the red and white blood cell counts.
Unless you're interested in pursuing more aggressive chemo treatments with a bigger hospital that has a board certified oncologist, I'm afraid that there may be nothing else that can be done for him other than attempting to manage his symptoms - for instance by treating vomiting with antinausea medicines or inappetance with appetite stimulants. If these things aren't working, it may be time to consider humane euthanasia, especially if his quality of life is not good.
I'm so sorry that you've got to go through this and potentially make that tough decision. Please let me know if I can answer any other specific questions!