"I heard very similar things when I was younger," wrote Alex Corbisiero, a former England prop who now is a rugby pundit for NBC Sports.
"Alfie keep working hard, you will have the last laugh and I promise this experience will make you a better person later in your life. Keep smiling."
Springboks great Mtawarira wrote: "Big fella you have my support. I walked this road too once upon a time when people silently bullied me about my size and questioned whether I was the correct age for my age group. Keep your head high and never let the haters get to you."
Nigel Owens, one of the rugby's most famous match officials, offered to referee one of Alfie's games and another Welshman, former international player Andy Powell, said he would do a coaching session at Oakdale.
"Alfie," wrote France flyhalf Matthieu Jalibert, in French, "I was told all my youth that I was too small, too skinny, too thin. When I read your dad's post, it breaks my heart. Never lose hope. Believe in your dreams. Your dad is your number one fan. I am the second."
As of Tuesday, Mark Pugsley's tweet had more than 220,000 likes.