he is not saying it wrong. he is saying it forward.

1. th is one sound, and it is made between the teeth. The tongue has to come out through the gap. Phonetics calls it an interdental fricative, which is a long way of saying the teeth have to get out of the way first.
2. f is the lip against the upper teeth. Almost the same noise, half the equipment. Swapping one for the other is common enough to have a name: th-fronting. Fronting, because the sound moves forward, off the teeth and onto the lip. It has been spreading across Britain since the eighties.
3. Children are the last to get th, somewhere around six years old, and the reason the literature gives is that they do not have all their adult teeth yet. Every kid says teef for a while. Most of them stop.
4. A cat gets thirty adult teeth and the full set is in by six months. Twelve incisors, four canines, ten premolars, four molars. He has had the hardware since he was half a year old.
5. So the equipment was never the problem. He just keeps opening his mouth too wide to use it.