What is that red tassel around his neck called?
That is the cord and cross of the Archconfraternity of St. Stephen.
Also seen here:
A nice little award that altar boys sometimes get as a motivational incentive. It’s called the Guild of St. Stephen usually, I think.
Otherwise it’s decoration. Some parishes use it just for this, I believe, but not many.
The medal is a sign of an Altar servers dedication to serving. There is a bronze medal for serving as a guild member for 1 year, silver for serving for 10 years and gold for serving for 50 years. There is also a military style medal for serving for 20 years that you don't have to be a member of the guild for.
I have been serving for 19 years and have only been in the guild for 2 years because my parish weren't involved until we got a new priest who had been in the guild.
Here is a link to the website guildofststephen.org/index.php?id=1
Originally it was British but it was spread to all english speaking countries.
Edit: added link