Foster chihuahua

I have found myself with a long haired chi for an undetermined amount of time
He's 3, neutered, totally unsocialised, not toilet trained, never been walked etc.
I have a very tolerant lab bitch already. She is generally great with other dogs and has pretty much ignored him since he's been here.
My plan is to treat him as I would a pup with regards to training/socialising but I've never had such a little dog before. I'm a Springer/labrador girl and don't want to get it wrong.

He's been treated purely as a wee lap dog and I don't even think he knows he's a dog.

I'd be very grateful for advice. Contacting the breeder isn't an option unfortunately.

Ignore his size he's a dog and needs to be treated as one. If you won't accept a behaviour from a big dog then don't accept it from a small one.

Your own dog is ignoring him for a reason, to let him know he isn't on the radar until his behaviour improves. Do the same, praise any good behaviour but don't give him undue attention, he has to work for it.
Read an article today about how many of these 'handbag dogs' are now in rescue, very sad and upsetting. The poor things

Thanks for the replies folks. Pretty much sums up what I've been doing with him.
He just seems such a sad, scared wee guy. Here's hoping he can learn how to be a dog

You've got to let him come out of it himself, he'll start responding to you, but don't push him.