Some puppies take longer than others to get the hang of toilet training, so don't worry, it's not too late and he'll get there eventually. I think winter puppies often take longer than summer puppies simply because the bad weather and dark evenings and mornings make going outside less attractive to puppies and owners
Go back to basics. Take him outside every time he wakes up, after eating or drinking or playing, and in between times watch him like a hawk for signs such as sniffing and circling. If he hasn't had a pee or poo after several minutes outside, take him back in and stay with him and watch him if that's the time when he's most likely to go. At the first sign that it's imminent take him straight back out again. Praise him when he gets it right.
When he pees or poops in the house
don't tell him off, not even a sigh of frustration. If you tell him off for peeing in front of you in the house he'll be wary about peeing in front of you in the garden. Result, a confused puppy who may think he should only pee when people aren't around or in hidden places like behind the sofa.
Use either a shop bought pet stain and odour remover, or a solution of biological laundry liquid or powder to clean up. The enzymes will remove the smell of pee whereas ammonia based cleaners and bleach smell like pee to him and will encourage him to use the same place again.
At 18 weeks he's probably still teething and may be grumpy and bitey. If he doesn't have lots of sleep and is on the go a lot he may also be overtired, which will also make him more likely to nip. I don't know what you've tried so far, but if giving him a large toy or a chew when he goes to bite isn't working, you could try walking out of the room without saying a word and leaving him for a minute.
You could also try quietly praising him and rewarding him when he's being nice and quiet. Not enough to get him over excited, but a gentle smooth and word of praise so he knows that being relaxed and quiet brings rewards.
Hope this gives you some useful ideas