Tietjens told the Samoa Observer the abrupt resignation of the team's last trainer led to him bringing in his son to take on the role.
Paul is a 28-year-old physical education teacher who was also the conditioning trainer for the Bay of Plenty Sevens team for the last two years.
Tietjens senior said with only four tournaments left in the world series, the team was in dire need of a trainer who already knew his expectations and had experience.
The veteran coach said when Hugh Slobbe left, the team was left without a conditioning trainer and in the last tournament, it was too much to try and coach the team and be their trainer at the same time.
Tietjens said the position would be advertised at the end of the year as it was too late in the season to do so now.
While the team had a new trainer in Hong Kong, it is also without Assistant Coach, Stephen Betham.
Betham, who coached the side to world series victory during the 2009/10 series, remained in Samoa this week.
Tietjens said he needed someone back on the island who was "actually pushing the players to the level they need to be at, fitness-wise."
"What's really important for me is the players that missed selection have someone back on the island still pushing them and working them hard.
"The best person to do that is Stephen because he understands the requirements."
Samoa face England, Australia and South Korea in the pool play this weekend.