"He is an opportunist, there's no need to give him more excuses or opportunities!"

I believe the idiom have an eye to/for the main chance comes close. It is more of a British idiom.

Someone who has an eye to/for the main chance is always ready to use a situation to their own advantage. Cambridge

As you can see from the definition, the expression emphasizes that one is always ready to take advantage of a situation and always looking for opportunities. He is basically opportunistic.

The origin of the phrase comes from gambling:

The origins of this expression lie in the gambling game of hazard, in which the person about to throw the dice calls out a number between five and nine. This number is called the main or the main chance, and if someone rolls it they have won.

_{Oxford Dictionary of Word Origins by Julia Cresswell}

Many a parent will say this to the other parent, when the other is thought to have been too generous or lenient with the children:

Give them an inch, and they'll take a mile.

It's used as a caution against the type of opportunism illustrated in the question. And there are several related forms, such as, "Give a fool an inch, and he'll take a mile," and "Give her an inch and she thinks she's a ruler."

Source: A Dictionary of American Proverbs By Wolfgang Mieder, Stewart A. Kingsbury, Kelsie B. Harder

Another possibility, cited from a collection of English proverbs:

He who handles a nettle tenderly is soonest stung. Figuratively, when one is dealing with troublesome people and affairs, the use of gentle methods will subject one to harsh and painful reaction, from which you will not suffer if you employ drastic measures. 1579, Lyly: True it is that hee which toucheth the nettle tenderly is soonest stoung. 1753, Aaron Hill, 'The Nettle's Lesson'-- Tender-handed stroke a nettle/ And it stings you for your pains;/ Grasp it like a man of mettle,/ And it soft as silk remains.

from A Book of English Proverbs by V.H. Collins, 1959.

"He already dances without music! Why did you give him such an excuse to escape from doing his assignment? You should have asked for his help after he had done his assignment!"

Rather prosaic but.

"He doesn't need any excuses to get out of doing his homework. You should have asked for his help after he had done his assignment!"

You can find other examples by Googling "need any excuses" - include the quotation marks.

While the idiom is not current, the King James Bible, Proverbs 26:17 says

He that passeth by, and meddleth with strife belonging not to him, is like one that taketh a dog by the ears.

So, from chasly's answer, such a person can be called a "meddler".

EDIT It occurs to me that, while I've never heard an English phrase which expresses

He doesn't need any special, real, or necessary excuses for taking advantage of a situation or to interfere in other people's affairs. He uses the smallest pretext for his unwanted intervention/taking advantage. Let alone you encourage him with more excuses or opportunities.

there is a phrase which implies it.

Don't encourage him. Spoken with a shake of the head, and often a tone of weary disgust.