Heavy flow period underwear are going to be your best option. They do make incontinence underwear for men as well, however I am not sure if the layout of them has pad that goes all the way back that would catch drainage.
Here are some disposable pad specifically for the backside:
B-Sure Anal Leakage Pads, Box/24 Pads https://www.amazon.com/dp/B005GDTWOG/ref=cm_sw_r_cp_api_i_NZ959SD7FMDX5CWW780K
Here are some reusable pads I hear great things about:
Lilind® 10 Pack Interlabial Pads, Reusable Organic cloth, Washable Menstrual Pads, Zero Waste 100% Cotton Random Flowers https://www.amazon.com/dp/B088T3SNCJ/ref=cm_sw_r_cp_api_i_T101W85SY4CYH1KD8VKR