As it has only happened twice and he is otherwise well, I would not be overly concerned.
Many dogs chew and eat grass and then throw up a small amount of bile or saliva.
There are many theories a to why this may happen. Some dogs like the taste of sweet grass in the summer while others may eat the grass to reduce bile secretion.
Going forward, stop letting him eat grass. If he is trying to, clip his lead back on and distract him with e.g. a game, some training commands or a toy.
If you use a neck collar, switch to a body harness as this puts less pressure on small necks.
Ensure Bentley is up to date with his intestinal worm and lungworm prevention.
Ensure he hasn't eating anything different recently; such as a new chew or rich human food. Try to keep his meals bland (just regular dog food) and to feed him morning and evening.
If his signs continue or worsen, it would be best to have him assessed by a vet in case there is more going on. Other considerations would include e.g. an infection, tracheal collapse or a foreign body in the throat.
I do hope this helps!