Peer pressure is indeed strong at this age. If the school will assist, it may be best to take advantage of it. My son was potty trained in no time with the assistance of day care and peer pressure. (Your son has had a big life change with a new sibling added to the family, this is sometimes tough on little ones. I recommend you not force the issue.)
Also to quote a notable Child Pschologist, Kenneth N. Condrell, Ph.D., "STOP TOILET TRAINING WHEN IT'S NOT WORKING Toilet training often becomes a battle of wills between parent and child. Remember, children of toilet training age usually have no interest in using the potty. They're also going through a stage of life when "No" is their favorite word.
Parents, on the other hand, are highly motivated to succeed at toilet training because it ultimately means less work for them. However, you have to consider your child's physical and mental maturity. In other words, if your child isn't mentally and physically ready, toilet training isn't going to happen. Trying to force a child to make it happen only delays the whole process.
So, if your child isn't toilet trained after your good effort, back off and try again in a month. When it comes to training a child to use the bathroom, sometimes waiting is the best approach."
I wish you the best!