My DS2 has a lisp and can't say his 'S' he is 4 in Aug this year. My SIL is a Speech Pathologist/Therapist and explained she wouldn't worry about as it's quite common. I too had a lisp as young one and it corrected itself in late Primary School.
On saying that I would talk to your MCHN or even GP to see if you can get a referral to a speech therapist to put your mind at ease and make sure this is normal for your own peace of mind.
I also rememember DS1 saying the rhyming word for Truck when he saw a truck until he was at least 4.5yrs old and it too corrected itself. Some letters and common sounds like 'TR' and 'S' are apparently very commonly mis pronounced by little ones.

Good luck

x Heather