Your child has now learnt about various food items. He has developed a good taste for various eatables. Let him eat his food on his own without any assistance.
Table manners are very important at this age. You need to teach the kid so that he can acquire good habits. Once the kid has learnt to use a spoon to eat food then the next step should be to introduce the use of fork and knife. This practice might require great focus.
What you can do
The first step should be to demonstrate it to the child. Show how to cut a slice with a knife and then use the fork for eating. The movement of the hand and the correct way to slice should be taught in an easy way. Let the child try it on his own and make mistakes. This is the time when you have to correct the kid to make him execute this task properly.