Non Payment of Service Amount

I am a tax consultant providing GST and Income Tax filing services.In 2021, a business consultant running a travel agency and software services firm approached me. He proposed that I provide tax filing services for his clients, while he would handle client interactions and payments. There was no written agreement between us, but I provided services regularly, and he paid for them until July 2024. Since July 2024, he has failed to pay my fees. Despite multiple reminders via WhatsApp, emails, and a registered letter, he has not cleared the outstanding amount.He responded once, vaguely alleging overcharging but has not provided any proof or engaged in any discussion to resolve the matter. Due to non-payment, I stopped my services in September 2024. My Queries: Can I file a legal case for recovery of my unpaid fees, even though there was no written contract? Can I include his clients as third parties in the case since they benefited from my services? Can I send a notice to his clients informing them that their tax filings were done by me but remain unpaid by the consultant?