It is absolutely clear from Act I scene 1 that, in spite of Bassanio's rather spendthrift ways and the way that arguably he could be said to be using Antonio rather manipulatively, Antonio would definitely give Bassanio the money he asks for if he had it. However, as has already been established in his conversation with Solanio and Salerio at the beginning of this scene, Antonio's wealth has been staked on various merchant ventures and he therefore has no present store of money that he can give his young friend. Note what he says to Bassanio in this scene:
Thou know'st that all my fortunes are at sea,
Neither have I money, nor commodity
To raise a present sum...
Antonio therefore, as a merchant, has staked all of his wealth on equipping and sending out various ships in order to gain goods from far off places that can be then sold to gain him wealth. Based on the historical context of the play it is important to remember that this was a time of massive mercantile expansion and tremendous wealth could be made if ships returned successfully, which is of course what eventually happens to Antonio. However, as is shown in this quote, such gains were made only at considerable risk: Antonio has placed all of his fortune on the hope that these ships will return, and thus he is unable to help his friend now. Therefore Bassanio is sent to borrow the money that he wants in Antonio's name instead, thus instigating the main conflict in the plot between Shylock and Antonio.