Summary and Recommendation
Given that he is poor, in the East, and has only 3 days, his options are severely limited.
Legal avenues are likely a dead end unless he has a major, documented illness.
Hiding or evading is a path to a life of paranoia and poverty with a high likelihood of eventual capture and harsher punishment.
Fleeing the country is prohibitively expensive and lethally dangerous.
The most pragmatic, albeit terrifying, strategy is likely #4: Comply and Survive. It is the only path that gives him a degree of control over his potential role. By reporting, he avoids becoming a criminal. By strategically presenting his skills, he can actively work to be placed in a non-combat or rear-echelon role, which drastically increases his chances of survival compared to being an infantryman in a trench.
It's a choice between a near-certain negative outcome (prison via evasion) or a high-risk gamble on survival within the system. For a person in his desperate situation, the latter offers a sliver of agency.

my personal opinion is that the best strat is to run to a different part of ukraine with whatever borrowed money you can get. take all money you can get immediately and run and hide. if you can somehow live in teh woods far away alone in some hole in the ground, its much better than having your face blown off for politicians ego

its not impossible that the war could end in 6 months. delaying you going to war could prove critical to your survival. same as going to jail if you can go to prison than war its much better

idk if your parents are supportive, but if you express you dont want to fight and die they might be willing to help