I have a 1.5 year old mastador who walks in a halti and is genrally well behaved unless he sees another dog. Then he rears up on his back legs and drops suddenly backwards managing to slip his halti completely. I have tried every adjustment to the harness but he still slips it, and he does this so quick I'm often too late to stop it, especially if he can tell a dog is near but I cant. Advice?

How about adding a collar and lead as well, or trying a different harness ?
My toy breed just wears a harness from P a H , but couldn’t slip out of it.

I agree withst Domingo  how about trying a different  harness as well as the halti if u think he needs the halti
The julius is very popular although I havnt used it as I prefer those with a front and chest/ side ring and they didn't do one .
Also look at different  head collars with   extra attachment to his collar

As well as fnding a better halti / harness for control I would suggest lots of work on his behaviour starting at a distance to help the situation.  Loads  are of rewards and no punishment of course.  Get his attention on you not just when things are likely to go wrong but generally

What kind of harness would you recommend? I've tried him with a couple before but he's a very string dig and harnesses just seem to allow him to drag me along. I didn't realise there are attachments for the halti so might look up online about those as well. Thanks for all the help so far!

i take it this is the head collar Halti?  if it is go buy a choke chain and clip your lead to both the head collar and the chain then if he slips the head collar you will still have him by the chain.  I am not saying walk him choke chain alone but use as an emergency extra .

Perfect fit , meckuti are both available with two attachments. Perfect fit are very helpful of u ring them and will give advice  and I believe julius k9 now does one in which u have the option of an extra ring on the chest of the harness

Personnaly the only harness I have found helpful is the two attachments as it allows control rather than the dog pulling u along

As for.the halti definitely have the extra attachment to the collar regardless of if u have a harness.
It will help u both if u concentrate on more training parfocilary the watch me command

Personally I don’t like the harnesses that rest on the shoulder joints (like the julius k9) as they restrict movement. Y shaped ones with a strap going from chest to belly are much better.

I have a Mekuti for River (papillon) and while pulling obviously isn’t so much of a problem with a 4kg pup, I really do notice a difference between using the back clip and the front clip.

On the front clip he just doesn’t even try to pull unless he’s super excited, and even then he can’t pull very much at all.

I used to use a Halti headcollar on my old Labrador and she learned to brace her neck so she could pull fairly hard anyway! And if we stopped for a minute she’d rub her muzzle on my leg to get the nose strap off.

Halti's imo are crap. Get yourself a dogmatic headcollar, much like a horse bridle and strong. Its what i use on my Rottweilers.
Re harnesses the Julius type are crap imo. They can slip back out of them, no front ring so you have zero control from middle of the back forward and they can back out of them.
Harnesses i recommend the perfect fit, padded in the right places ans have a front ring for a double ended lead.
Personally if the main issue is control then a dogmatic i would opt for over a harness.
Also i would look at a 121 trainer or classes as this needs to be worked on in a controlled environment too with help

I agree, get a dogmatic and another attachment point like a harness. Perfect fit would be a good one, or ruffwear, dog Copenhagen or similar designs. Julius K9 are awful harnesses and they can slip out of them too. Don’t get anything that restricts shoulder movement.
And obviously.... these are just tools to compliment effective training