No dose adjustment is necessary with antacids, proton-pump inhibitors or histamine H2 receptor antagonists.
Telzir Summary of Product Characteristics, ViiV Healthcare UK Ltd, January 2021.

Proton pump inhibitors can be administered at the same time as a dose of fosamprenavir with no change in plasma amprenavir concentrations. Coadministration of esomeprazole (20 mg once daily for 2 weeks) and fosamprenavir (1400 mg once daily for 2 weeks) to 25 subjects resulted in no change in amprenavir Cmax, AUC or Cmin. When the same dose of esomeprazole was given to 23 subjects receiving fosamprenavir/ritonavir (700/100 mg twice daily), there was no effect on any amprenavir PK parameter. Esomeprazole Cmax did not change in the presence of fosamprenavir or fosamprenavir/ritonavir; AUC increased 55% in the presence of fosamprenavir alone, but did not change in the presence of fosamprenavir/ritonavir.
Lexiva Prescribing Information, ViiV Healthcare, October 2020.

In a single dose study in healthy volunteers, when fosamprenavir (1400 mg) was administered 1 h after the H2-receptor antagonist ranitidine (300 mg), amprenavir AUC and Cmin were decreased by 30% and 51% respectively; there was no siginificant effect on amprenavir Cmin. Caution is recommended when fosamprenavir is coadministered with H2 receptor antagonists or proton pump inhibitors (such as lansoprazole).
Effect of antacids and ranitidine on the single-dose pharmacokinetics of fosamprenavir. Ford SL, Wire MB, Lou Y, Baker KL, Stein DS. Antimicrob Agents Chemother, 2005, 49: 467-469.