NOTE: I'm not a lawyer
Can I get a joint name letter of all 3 other tenants (includes me), so that if he doesn't stop smoking in the property he will have to move?
From this line, it sounds like you do not own the property. A joint letter will be absolutely useless. Here's why
and in my contract, there isn't specifically a part about no smoking
Given this he could smoke in the house. He's being a good house-mate by smoking outside in the yard.
I have discussed this with my other 2 tenants before he moved in they were OK with it
And the landlord did the right thing and asked the current tenants about the new tenant. They all said smoking was ok BEFORE the new tenant spent the money and time to move in.
Now my other 2 tenants complain that even this is not enough, they want to him to either smoke outside of the property or evict him.
There is likely no legal way to do this. The problem tenant signed a lease with the landlord - not with the other tenants. Even if this is a co-living situation (or you are the landlord), the house-mates already gave the go-ahead knowing he smokes. Now they want to renege.
Assuming every house-mate signed a lease, they can always choose not to renew when their lease is up. Moving forward, be sure to put language about smoking habits in the lease. Until then, you're likely stuck with the smoker.
About the only thing I could think to do here is have the non-smoking room-mates pool money and essentially bribe the smoker to move. You'll have to live with the guy if he says no - so be sure this is a hill you want to die on.
Some comments have questioned if the landlord is legally responsible for ill-health caused by 2nd hand smoke. Especially if he is smoking outside, I doubt this would be a cut-n-dry case, and likely involve hefty legal fees.