A: Hi there!
The secret of teaching is to gently stretch the child "one step more." If he already knows counting to 50 then you add 51. That's the basic premise. So the first step is to be sure what you are teaching is within his realm then slowly introduce new facts
Second - make it fun!! You're frustrated but can you imagine how he feels? Make things into a game. Hide the numbers -- go find them. Hold a scaverger hunt. Use chalk and write them on the sidewalk so he can jump to them. Keep things active. Sing jingles about them. Anything! Fingerpaint the numbers in pudding! Just keep it light. He's probably feeling frustrated-and that stress can reduce focusing abilitiies