Can you take him to you MCHN (if you have a good one)? They see heaps of kids that age, and so will be able to give you some insight into whether his speech is outside of normal, and then refer you for speech therapy if it's needed.
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.
DD1 is 3 yrs 7 mnths and is the same. She gets her tenses mixed all the time and says things the wrong way round. I think she will grow out of it. I was told to not correct them as in "don't say it like that but like this". You should rather just follow their word with the correct one. eg son says "This is a wower" and you say "yes, this is a shower" otherwise they lose confidence. Small kids can't say S and Sh so well but it comes with practice. He sounds normal to me but ask someone professional even if just to put your mind at rest.
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