Would the son be sinful if he stopped supporting his parents' losing projects and focused on investing in his own projects to ensure his ability to meet the needs of his family and himself, after having supported his parents' projects for five years?

You bear no sin for not supporting your parents' business ventures. What is incumbent upon a child towards their parents is to spend on them to the extent of their need if the child is able and the parents are in hardship. It is not required of him to spend all his money and remain empty-handed. Nor is it befitting for parents to burden their children with what is difficult for them.