I read the problem about: "A Speech Impediment "
Your child is still VERY young. At this point I would say don't worry about it. If this still persists by 2nd or 3rd grade it MIGHT be a concern, but certainly not at age 5, if there's only the one sound he has difficulty with.
If he has a good vocabulary and is able to communicate effectively, difficulty with one initial consonant sound isn't a big deal at this age. It's entirely "normal" for a child to have trouble with one or a few sounds, especially initial consonants & blends, right up through 2nd or 3rd grade. Usually this will correct itself naturally as the child grows and becomes more confident with spoken language. However if a child is made to feel self-conscious or is punished for it, the speech impairment often becomes worse.
At all costs, avoid pressuring him about this or making him feel you're disappointed with him. With the best of intentions, these days we put so much pressure on kids to perform and to conform to often-unrealistic standards... we need to remember that a child is a child, and the developmental process is a very individual thing! Sorry for the soapbox (& please don't take any of it personally) but I see this happening so much and it's very sad!
With all of that said, IF you feel you can do this without pressuring your little one, you might try making little games that involve using the initial "f" sound. Practice some of the words that give him trouble yourself, in front of a mirror so you can see the correct way to shape the sound with lips, teeth, tongue, etc. Then you can model this for him. Make up little rhyming and/or alliterative songs, chants, or raps that use initial "f". Again, only do this if you can manage it without making it stressful for both of you! It's OK to reward him for pronouncing the sound correctly, but don't chastise him for doing it incorrectly .
