TACHA, Chief Circuit Judge.
Plaintiff-Appellant John Proctor appeals the District Court’s entry of summary judgment in favor of Defendant-Appellee United Parcel Service (UPS) on his claims that UPS terminated him in retaliation for filing administrative charges of disability discrimination in violation of the American with Disabilities Act (ADA), 42 U.S.C. § 12203(a), and for filing workers’ compensation claims, in violation of Kansas law. We have jurisdiction under 28 U.S.C. § 1291 and AFFIRM.
I. BACKGROUND
Mr. Proctor was employed by UPS as a package car driver, a position that requires an employee to lift, lower, leverage, and manipulate packages weighing up to seventy pounds and to work extended hours when necessary. In February 1998, he underwent surgery on his wrist for a job-related injury and eventually returned to work. In October 1999, he sustained a job-related injury to his back, after which he was examined by Dr. Fevurly, UPS’s doctor, who released him to return to work with no restrictions on November 8, 1999. One week later, he suffered another work-related injury to his back. Mr. Proctor pursued workers’ compensation claims based on his injuries, filing the latest of these claims in November 1999.
The following month, in December 1999, Mr. Proctor returned to UPS for a brief period after Dr. Fevurly released him to return to work with a lifting restriction of thirty pounds. His last day working at UPS was December 24, 1999. From January 2000 to July 2000, Mr. Proctor was examined by Dr. Fevurly several times. In July 2000, Dr. Fevurly concluded that he had reached “maximum medical improvement” and imposed a permanent fifty-pound lifting restriction.
According to Mr. Proctor, he was subject to medical restrictions that prevented him from performing his job duties until January 2002. During this time, Mr. Proctor continued to be treated by his own physicians in addition to seeing Dr. Fevurly. His back doctor, Dr. Prostic, released him to return to work with no restrictions on February 1, 2002, and his wrist doctor, Dr. Ketchum, issued a full release on April 3, 2002. When UPS did not allow Mr. Proctor to return to work despite his doctors’ releases, he filed a grievance against UPS for violating the collective bargaining agreement (CBA) between UPS and Mr. Proctor’s union, the International Brotherhood of Teamsters, Local Union No. 696 (Union). Under the CBA, after an employee presents UPS with a doctor’s return-to-work slip, the company doctor must examine the employee within three working days. After Mr. Proctor filed the grievance, the company doctor, Dr. Fevurly, examined Mr. Proctor and did not release him to return to work, concluding that his work status was “to be determined.”
Under the CBA, when UPS’s doctor (i.e., Dr. Fevurly) and an employee’s doctor disagree, UPS and the Union must agree on a third doctor whose decision is “final and binding” on the employer, the Union, and the employee. Pursuant to this provision of the CBA, on April 30, 2002, Dr. Brown, the doctor selected by UPS and the Union, examined Mr. Proctor and imposed a lifting restriction of forty pounds. In addition, in his medical evalúa