"He has a son from this relationship."
In German the word order of the sentence is quite flexible, but there is one important rule - the verb is always the second 'idea' in the sentence (unless you are asking a question, then it is the first). In this sentence, 'Aus dieser Beziehung' is one 'idea' so 'hat' (the verb) comes next. You could also say 'Er hat einen Sohn aus dieser Beziehung' or 'Einen Sohn hat er aus dieser Beziehung' but the verb 'hat' is always the second part of the sentence. In 'Aus dieser Beziehung er hat einen Sohn' the verb would be the third idea (after 'Aus dieser Beziehung' and 'er'), so it wouldn't be correct.