If the two substitutions you give as examples are the only ones he makes, I wouldn’t be at all concerned. The “F” and “S” are both called fricatives, and they are difficult ones for young children to make. Of course, since he now substitutes “F” for “S,” we know he can make the “F” sound. It’s just a matter of getting the sound into the right word.
Substitutions for the “S” sound are among the ones we traditionally call “baby talk” (“I tought I thaw a puthy cat”). Certainly you and your husband can work with him at home and provide a significant amount of help.
I’d start with the “F” sound, not only because he can already make it but also because the way to make it can be visualized. Sit in front of a mirror and say “F.” You will note that, in order to make it, you must touch your lower front teeth with your upper lip. Now try “S.” You can’t see what you did, can you? So get your son to look at you and say, “Watch what my mouth does when I say ’fish.’” (Say it, exaggerating the “F” sound.) “Now you say it.” Don’t do the exercise just with “fish,” the word the teacher and therapist say he has a problem with. Try other simple words he knows—“funny,” “far,” “fat,” “fast,” “food,” “family,” etc.—and help him realize that he must make the “F” sound to produce all those words.
Do the same thing with “S” words. Let him see that, to make the sound, the mouth must be slightly open, the teeth barely parted, the tongue just behind and that then he has to blow. This is harder to see, but with practice he will get it. Try it with any number of simple words—“see,” “slow,” “summer,” “Sally,” “Sunday,” etc. Hold off on words like “Spiderman,” as the consonant blend (“Sp”) is harder than the simple consonant (“S”).
The most important thing in all this is not to make your son anxious about talking. When you do these things with him, make it a game. Make it fun, but don’t laugh at him when he makes a mistake. Don’t ask him to repeat everything he says wrong. And, when he reaches the stage at which he no longer makes the substitution, praise him casually with something like, “Hey, you said ’Spiderman’ just right.” If, when he is 4 years old, he still makes all the substitutions, then I would look into speech therapy for him.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.