It's 5 Shekels. Period. If you don't have it you can't do the Mitzva just like if you were stuck somewhere on Sukkot without an Etrog. Sad, but that's the way it goes. (ShA YD 305:1,6)
They cannot even have the Kohein collect from lands they sold after the son was born and debt took effect (:17).
If the father or son ever do manage to get 5 Shekels (even after one of their deaths (:12)) they still have a biblical requirement to give it to a Kohein (:15).
One trick which might help is giving an item which isn't worth 5 Shekels in the market (but is worth that much to someone) to the Kohein and having him declare he is accepting it as 5 Shekels of payment (:5). (The details of this are tricky so CYLOR.)
If the father manages to get 5 Shekels first and he also needs to be redeemed himself, he redeems himself first (:15).
If the father/son is very poor the Kohein is allowed to give back the money after the redemption and not worry that his doing so will encourage others to use him as their Kohein thereby depriving his brethren of income (:8).