i'm sure this must be somewhere in the super long thread of proverbs sayings etc, but will ask:

what is the equivalent proverb for:

beggars cant be choosers

(telling boyfriend he has to deal with the work available not what he wants) )

There's a rather direct translation in my Oxford-Duden:

ผู้ที่เป็นยาจกไม่มีสิทธิเลือกได้ phuu[F] thii[F] pen[M] yaa[M]jok[L] mai[F] mii[M] sit[L] leuak[F] dai[F]

(person [rel.pronoun] be beggar no have right choose can)

Not sure how it will be received. If he is not familiar with the saying, he might be insulted by your insinuating he is a beggar.

As a side note, Swedish has "You have to adapt your mouth to the provisions at hand" and German has "Any bread tastes good in times of hunger"... there are many ways of expressing this, the beggar implication is the most likely for causing insult.

Why not simply say: "In your situation, one cannot afford to choose".

ใน สถานการณ์เช่นนี้ไม่มีสิทธิเลือกได้

Before you lay the "beggars can't be choosers " line on him, make sure you'll know how he might react. If I were to say that to my girlfriend for instance, whatever I have been meaning to convey would be out the window and we'd immediately be discussing "oh, I'm a beggar am I?" "Is that how you see me, as a beggar?" It's a minefield out there.

ผู้ที่เป็นยาจกไม่มีสิทธิเลือกได้ phuu[F] thii[F] pen[M] yaa[M]jok[L] mai[F] mii[M] sit[L] leuak[F] dai[F]

I am confused by ยาจก in the above sentence. Can't find this word in my (cheap) dictionary!

Can you use ขอทาน instead for this saying?

Yep, ขอทาน is the common word for 'beggar' so it should be fine, although the style is affected somewhat. You will also see กระยาจก with the same meaning...