Laffoon, 20, filed his opposition against on Tuesday claiming Tupper — who is Atlas's biological father — has "conflicts of interest" in becoming the 13-year-old's legal guardian and that his appointment in this circumstance "would actually harm the interests of [Atlas]."
In the filing obtained by PEOPLE, Laffoon (whom Heche shared with ex-husband Coleman Laffoon) claims Tupper is "precluded from serving as the minor's guardian ad litem based on several actual and potential conflicts of interest."
Noting that Tupper has acted as a father figure for both Laffoon and Atlas, Laffoon claims that relationship could pose problems if he's "ever forced to side with one child against the other."
getty (2); Anne Heche/Instagram
"Dependent upon Mr. Tupper's intentions and actions regarding these funds," the document continues, "it may be necessary for the Estate to institute legal proceedings against Mr. Tupper for the ultimate benefit of [Atlas]. As such, Mr. Tupper has an actual conflict of interest with [Atlas] regarding these funds."
Laffoon's filing was in response to paperwork filed by Tupper on Monday.
The filing, which PEOPLE has obtained, claims "ATLUS [sic] prefers a bonded, neutral, private, professional fiduciary be selected to administer the subject estate. As a minor, ATLAS has no ability to hire legal representation to represent his interests in these proceedings."
The document notes that Tupper, 57, "is ATLAS' father and only living parent. JAMES loves both HOMER and ATLAS as a father and wants the best for them both."
It continues, "In order to preserve family harmony and a healthy, brotherly relationship between ATLAS and HOMER, and given the complexity this estate will foreseeably involve – i.e. intellectual property and publishing issues, possibly third party claims, and public relations issues – a bonded, neutral, private professional fiduciary would be a more appropriate administrator."
Never miss a story — sign up for PEOPLE's free weekly newsletter to get the biggest news of the week delivered to your inbox every Friday.
Heche died after being involved in a fiery car accident in Los Angeles on Aug. 5. After being in a coma, the state of California declared Heche legally dead on Aug. 12. She was temporarily kept on life support in order to donate her organs. On Aug. 14, her rep confirmed to PEOPLE she had been taken off of life support.
At the time of Heche's death, it did not appear the actress had written a will. However, Tupper has since surfaced an email Heche sent to him and two attorneys in 2011. The email requested that her estate to go to Tupper and assets be divided between her two children when they turned 25.
Laffoon has filed against the will's legitimacy on the ground that it does not have an authentic signature and was not signed with two witnesses present.