Well, each of these drugs has a contraindication against their being used together. I've dealt with people online who have had this happen to their cats and some of those cats have experienced what the Convenia manufacturer describes as a severe adverse reaction. People have lost their cats because of this.
Some of the reactions and the necessary interventions are described in the drug insert:
CONVENIA is contraindicated in dogs and cats with known allergy to cefovecin or toβ-lactam (penicillins and cephalosporins) group antimicrobials. Anaphylaxis has been reported with the use of this product in foreign market experience. If an allergic reaction or anaphylaxis occurs,
CONVENIA should not be administered again and appropriate therapy should be instituted. Anaphylaxis may require treatment with epinephrine and other emergency measures, including oxygen, intravenous fluids, intravenous antihistamine, corticosteroids, and airway management, as clinically indicated. Adverse reactions may require prolonged treatment due to the prolonged systemic drug clearance (65 days)
(I've bolded emergency responses)
FOREIGN MARKET EXPERIENCE:
The following adverse events were reported voluntarily during post-approval use of the product in dogs and cats in foreign markets: death, tremors/ataxia, seizures, anaphylaxis, acute pulmonary edema, facial edema, injection site reactions (alopecia, scabs, necrosis, and erythema), hemolytic anemia, salivation, pruritus, lethargy, vomiting, diarrhea, and inappetance.
What's done is done.....my only advise on this is that you watch him like a hawk and that you do everything you can to have him increase his water/liquid intake (I'd ditch the "dry biscuits" - maybe just use crumbs). And, if it were me, I'd not be using any more Metacam. Certainly, if any of the adverse reaction symptoms develop, you would need to take him in and have appropriate treatment(s) started (I bolded those in the first quoted link).
There has been (at least) one very serious similar case reported on this forum....strangely, I can't find it. An apparent less critical incident was reported here: Is my cat in pain?
Unfortunately, the outcome is unknown as the member hasn't returned to update the incident. (The shivering she reported there was most likely due to anaemia.)
Do keep us updated......thinking of you both