Praise be to Allah.
If someone does work for someone else – whether he is a partner of his or not – without any agreement on wages, then in principle he is doing it voluntarily, and it is not permissible for him to demand anything if that work is not his career or he is not known to earn money from doing such work. But if that is the case, he should be paid the going rate, so it should be found out how much someone of his calibre would earn for supervising construction, and he may be given something similar.
It says in Kashshaf al-Qina‘ (4/206): Whoever does work for someone else without agreeing on payment is not entitled to anything, because he gave his services without agreeing on something in return, so he does not deserve anything, so no one will be obliged to pay something that he has not committed himself to and that he is not happy to pay. This applies if it is not the usual occupation of the one who is doing that work for which he usually charges a fee. If it is his usual occupation – such as a ferryman, supplier of hired mounts, cupper, dyer, tailor, broker and the like, or such as a shepherd, one who measures volumes and weighs goods, and the like, whose time is devoted to earning an income by doing this job, and the person who sought his help gave him permission to work, then he is entitled to payment at the going rate, as indicated by custom. End quote.
What appears to be the case from your question is that your partner is not known for supervising construction in return for payment. If that is indeed the case, then it is not permissible for him to demand anything.
Moreover, where has he been during the last few years, and what prevented him from making an agreement regarding payment?
And Allah knows best.
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