Ok it sounds and fells like he is not getting enough heparin during the tx. What leads to believe this is the slow rise in the venous pressure. How does his dialyzer and blood line look after TX. Did you push 1cc of normal saline on the first cannulation?
Exercise the AV/f using one, two, three, four five PDS weight devolved by dialysis Joe in VA hospital. Joe swears by this method as well as I have seen results. After two weeks and starting in the third week start using a one Pound weight like you are pounding nails in to a wall. The four week you move up to the two pds weight and one pds per week thereafter. if you have ever seen a carpters arm you will see that they have large well devolved veins that’s because the use of the arm. Do ten reps ten times per day this should help to developed the vessel a lot faster.