...Turns out his advice was very wrong!
Good thing you have realized that. That advice is unfortunately fairly common, but it is dreadful and foolish - new bird owners can't be expected to know, but people who SHOULD know better are the ones who are dishing out this stupid idea - they need to get "flicked" in the face a few times.
Is there any way to fix our relationship?
Good news is yes, it can always be fixed. But as Ecclipse mentioned it will take time and patience. It is easier to deal with a problem caused by a previous owner, here you are the one who (through no fault of your own) caused the problem - because of that it will take a little longer. Your senegal needs to not only learn that people aren't mean, but that despite what has happened before YOU aren't mean.
Birds can forgive - but they do make sure we learn our lesson first.
The best thing you can do if he bites now is NOTHING. At least not immediately. He should see that his bites do nothing, they get no reaction good or bad. If he does bite do your best to not even flinch or get angry, but you can calmly stand up, bring him to his cage, and give him a time out for a couple of minutes (yes just a minute or two). Put him in his cage and leave the room. When you come back let him out (provided he is reasonably calm and not screaming for attention) and act like nothing happened. There is a 'time out' punishment, but there should be no anger or grudge holding - once the time out is over it's like nothing bad happened at all and life goes on.
Let him learn by experience that you no longer react by flicking him on the beak. Let him also learn that you are patient and always willing to offer treats and good stuff. You will establish rules and enforce them but you wont get angry or vengeful. You will also not force him to come to you, but you will wait for him to come to you.