Advice on how to tell a child their daddy isnt interested

Hi my girlfriend's 3 and a half year old boy sometimes crys for his daddy and wants to see him but the problem is the father had drifted in and out of his life, often vanishes and leaves my girlfriend to pick up the pieces and when he did actually see him it was only for 6 hours on the occasional sunday and the father took him to his mothers and sisters so bonding contact was limited, if at-all.
A few weeks ago the father said he would start have him on a friday evening and over night until the saturday morning, even though this was a pathetic amount of time the child was really exited about it. Now the child support agency have caught up with the father he has said he is not interested in seeing his son again and my girlfriend just doesnt know how to tell him that his daddy wont be around anymore

Can anyone offer a bit of advice??

I try and be a father figure to him but ive only known him a few months and obviously i can never be his real dad

Unfortunately, I'm not sure that you can tell him. It is probably better to leave him to work it out for himself. If you or your girlfriend are seen to be telling him that his father won't be around then it may cause problems later in life. I know the father of my ex's first is a waste of space, but has at least managed regular contact. But she is starting to learn for herself that he is a waste of space.
Normally the CSA will cause an uninterested father to visit more as they feel like they are paying for the privelege, but they have to pay regardless of whether they visit the child or not.
Give it some time and just be there to support him. Hope it works out.