An exact answer may be impossible but here is some math to suggest the closest possible estimation.
First lets compare by arm length
Henry Cavill is 6'1" via his imdb page. So given the length of his body in pixels to the length of doomsday's arm in pixels we have:
D = Doom:510px
S = Superman:342px
C = Cavill Ht in Inches: 73
S/C = 4.698 pixels per ...
In the leaked script, which seems pretty accurate from a skim and after watching the film, he's noted as being in his 30's. As this comes from the leaked script though there may have been changes since so it might not be totally accurate.
CLOSE ON JOKER (30's), tears in his eyes from laughing so hard. He's trying to get it under control. His greasy, black ...
His mother worked for Wayne "30 years ago". Arthur says that while bathing her. So he's probably in his early to mid thirties.
Arthur Fleck: You worked for him over 30 years ago. What makes you think he would help you?
The true answer may involve a personal matter which will never be exactly known. Fortunately, the OP's actual problem is understanding why this gratitude would even exist. They ask, "So what could it be? To this question, there is a very logical answer involving Aquaman's Newfoundland production unit:
Hypothesis 1: There happens to be an accident while ...