Synthesis of a lactone
Here is a way to make the lactone:


To your aldehyde in dry THF at 0 °C, add the commercially available 3-butenylmagnesium bromide. Work up and collect the resulting secondary alcohol.
React the alcohol with dihydropyran in dry $\ce{CH2Cl2}$ to form the THP ether.
React this THP ether with $\ce{BH3.THF}$ as you did before, but do an oxidative work up with PDC to get the carboxylic acid according to Brown et al. [1], or do the basic peroxide as before to get the aldehyde, then oxidise that.
Treat the product acid with PTSA in MeOH to remove the pyran ether, the lactone will form spontaneously.


Your choice of alcohol protecting group will be driven by the oxidation conditions you use for the borane. THPO is known to be stable to PDC/$\ce{CrO3}$ in pyridine.

Reference


Brown, H. C.; Kulkarni, S. V.; Khanna, V. V.; Patil, V. D.; Racherla, U. S. Organoboranes for Synthesis. 14. Convenient Procedures for the Direct Oxidation of Organoboranes from Terminal Alkenes to Carboxylic Acids. J. Org. Chem. 1992, 57 (23), 6173–6177. DOI: 10.1021/jo00049a024.