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File an application U/s 151 CPC before Court to call the Dr. as witness stating reason as; "child health is concern for me and wife not co-operating to tell me who is his natural father about the health of child and Dr. who treated my child upon enquiring is giving gullible answers under the influence of mother of child who does not wish me the father to know health and welfare of child, hence I would like to elicit on record health information from the treating dr. for larger interest and welfare of child, hence allow accordingly."
Upon dr. appearing as witness ask him 1001 que. to fatherly satisfaction but you should know few medical terminology especially related to child (common) disease and treatments pertaining to age of a child.
However, before filing above send a Notice to Produce Form as per CPC to mother of Child with 7 days notice for reply. If she does not reply in 7 days then file above urgency application and request Court to dispose or dismiss either way on admission hearing itself (stress this) it shows you as a father’s concerns otherwise it may be a frivolous (back fire) upon you !