Depending on his income, they might question the amount. €30 a week is very low.
I think you need to ask him his income, and come to a fairer amount than €30. After 4 years together he should be ok with disclosing this information to you. Have SW sought maintenance from the father of your older children also? It would seem unfair that they are only targeting your current partner.
It is unusual, normally at this juncture in your relationship, the partners would be seeking to form a family with all that entails, a shared home, shared responsibility for the baby etc. There is probably a lot more to this story than you can, or indeed should tell here, but I think you need to get this man to step up to his responsibilities a bit more.