Thank you for the question.
How many years has your client been powerlifting for?
Has he benched in a shirt before?
What type of training have you been prescribing?
How tight is the shirt?
Brand of shirt i.e Titan , Inzer?
Lots of factors have to be taken into account.
Depending on lifting age, I preferred to train in the shirt twice before the contest and max would be 3 times, but I lifted for many years and knew what weight I needed to get the bar down and what grove it should be in.
I have known some lifters to train in a shirt 6 – 8 weeks out from a contest. There complete cycle was in a shirt.
Novice lifters can take a dozen work outs in a shirt to get the bar to the chest or even get a lift that would be pasted in a contest.
I found putting the shirt work in after speed bench worked the best for me.
Benching in a shirt purely makes it a tricep movement, so train the Triceps very heavy and hard.
The key tip I can give you is don’t let the shirt change your grove, put you bench groove into the shirt not the other way round. A bench shirt is not comfortable and can cause a lot of pain and discomfort. But we do what we do because we love it! Pain does not enter into it.
You might find training with bench blocks or phone books might help him adjust. Start high and work into the shirt. i.e Start on a 6 block and work down to a single block and then no block. Keep the reps low, if any. singles will help.
The reason behind the shirt is yes you can bench more, but the main reason was that master lifters in powerlifting with years of low bar squatting and benching had screwed shoulders so it was a way they could start benching again.
I new a top NZ master powerlifter who once told me that the bench press was just a rest between the squat and deadlift and the contest did not start until the bar touch the floor.
But now, things have changed there are lifters out there that bench more than they deadlift.
If you can answer those top questions on the post I can go into more detail
I hope this helps,