If i say そいつは笑える would it be translated to "that's funny" or " that would have made him laugh" ?
In 99% of the cases, そいつは笑える (literally "That's laughable") should mean "That's funny". Another possibility, although a very small possibility, is "That person is able to laugh".
There is no way it can be interpreted as "That would have made him laugh" because there is no causative sense nor subjunctive mood in this sentence.
It would be translated as “He’ll be able to laugh.” or sometimes even just as “He can laugh.”
Because you’re using a verb with particle は it’s automatically implied that you say that whatever comes before the は performs the action (in most cases).
To say “That’s funny.” I suggest you use “それは面白い。”
Also keep in mind that you can only use そいつ and the like for informal situations. In formal situations you’ll hear そちら instead.
そいつ can mean a person and things. It is a rude way of saying a person and another way of saying それ.
そいつは笑える can mean "That's funny" and "He(She) is laughable", but it would be more used for things than a person. あいつは笑える is used for a person.
Depending on context, そいつ can refer to a person. そいつ can also be a substitute それ, when referring to things in a very informal way.
そいつは笑える can mean "that person's funny" or "that thing (tangible or not) is funny", with そいつ being very informal (and playfully endearing or purposefully demeaning, heavily depending on context)
More literally and generally: "that (noun) is laughable".
From my observations
When そいつ refers to a person/animal
そいつ/こいつ/あいつ contains a very colloquial, possibly endearing nuance: (in the case of a guy) "Oh, that dude? That fool's funny".
However, it may in the more literal sense be the opposite of endearing: "Oh, that guy? He's such a fool, it's funny (in a very condescending way).
そいつ/こいつ/あいつ is used by someone who considers himself/herself socially higher than そいつ/こいつ/あいつ in the context of what the speaker of saying. On the surface (like when お前 is used) this may be purposefully condescending, like when someone means to demean someone, but this can be also used as a term of endearment to signify that two people may have a good relationship. For example: two friends, a 先輩 referring to a 後輩 endearingly, or a pet owner to a dear pet. In modern English, it has similarities to when someone refers to a friend as "that fool", but in an endearing sense.
When そいつ functions like それ, referring to a thing
This is very similar to when a teacher, especially in science or math, refers to an entity (variable, symbol, etc.) as "this guy" or "that guy", in that a thing is being referred to in a light-hearted, casual way.
そいつ is an abbreviation for そやつ or そのやつ, used both for persons or for それ in limited situations: https://ja.wiktionary.org/wiki/%E3%81%9D%E3%81%84%E3%81%A4
笑える in this sense is one of those "translation traps" if doing a literal translation from Japanese to English, as common usage is different between the two. True, 笑える is the 可能形 of 笑う, but in English this may lead one to see this as strictly "able to laugh". But in Japanese, 笑える, at least in everyday usage, is used in situations where the English equivalent is "that's funny", often (not always) in situations that are kind of funny but not roll-on-the floor funny. If someone thought something was super funny, there are expressions like "HAHAHAHAH", "面白い!" "それは超面白い！" "あいつやばい!", etc.
そいつは笑える is translated to "That's funny", or "That makes me laugh".
It's commonly used to refer to a situation, and barely used for a thing or a person.
e.g. 「ガス欠だ」「そいつは笑える」 -> "I ran out of gas" "That makes me laugh"
-える(得る) is a verb that translates to "to obtain" or, in this case, "to be able to (do something)", and when used in conjunction with another verb e.g. 笑う(to laugh) it's used similarly to an adjective (technically it's not an adjective though).
笑える is "laughable", "able to make (someone) laugh". In a more literal translation, the phrase would be "That is able to make (me) laugh".
You may also use 笑える+noun to say that something makes you laugh. e.g. 笑える映画 is "Funny movie" or "Movie that makes (someone) laugh".