kitten litter training

hiya,i need advice,our kitten isn't always using his litter tray

The very first thing you need to do is clean every area where the kitten peed or pooped with Nature's Miracle or the like to competely eradicate the smell.
   
Even if you can't smell it, he can and it is like a beacon screaming to him, "PEE HERE, POOP HERE" that he cannot resist. This is why animals soil in the same places over and over again.
   
Ideally the kitten should have seen the mother squat and unload in the box and this is his clue to do likewise.
   
When he misses, how close is he to the box? Is he soiling near the box, or just going all over the house?
   
If he's going all over the house (and it's all pee, not poop) then it's possible he has a urinary tract infection. If that is the case, he needs a vet for an antibiotic.
   
This would account for wetting all over but pooping appropriately.
   
If he's pooping outside the box, too, then this is not your problem.
   
If he's going near the box but not in it, he could be having some trouble getting in and out of the box. Try a box with lower sides or use the low-sided box that cases of beer come in.
   
This issue should be temporary because as the cat grows, he will be able to get into the box with no problem.
   
Do you own other cats or is this kitten your sole cat?
   
If you own other cats, are they also using the same box or do you have several boxes?
   
If you have multiple cats, then this kitten might be uncomfortable using a box that other cats used so he's squatting elsewhere out of desperation; he has excluded himself from using the group box, but needs to potty so he goes where he is.
   
If this is the case, then you need to add a box and/or clean more frequently. When I lived in a multiple cat household, our rule of thumb was a ratio of 2-1, meaning for every 2 cats we had, we had 1 box. In the early going we had one that wouldn't use the box, but once we added the second box she fell in line.
   
Hopefully your cat won't be so much of a priss that he needs his own box. The problem with a multiple cat household is that it's impossible to give one cat a personal litter box because the other cats won't understand that one box is for that one cat's personal use and will continue to squat in it.
   
If he is the only cat, then it could be that you are not keeping the box clean enough for him. Is he using the box when it's freshly changed but squatting elsewhere once it's been used for awhile?
   
If that is the case, then he is telling you that he's a clean freak who won't use the box unless it is pristine, so you'll have to start scooping after his every pee and poop or he'll go elsewhere when the box isn't up to his exacting standards.
   
If his 'misses' are after the box has been freshly changed, then your problem is that by cleaning the box you are removing the smell of his pee and poop and thus removing his signal for where he's supposed to go.
   
In this scenario, he isn't sharp enough to figure out that he's supposed to use the box all the time, just not when he smells himself there, so when you change the litter, scoop a turd out of the previous mess and toss it into the clean box to give him a clue.
   
Good luck, hope one of these scenarios hits the nail on the head and your kitten is soon back on the straight and narrow.