As others have said its backdated to the day of the award. If your son has no other income in the meantime he can apply for supplementary welfare allowance from the community welfare officer in the HSE clinic. Also, depending on the household income you may be entitled to secondary benefits like free tv license, fuel, and telephone benefits. Apart from his free travel, he may also engage in voluntary and rehabilitative work while claiming disability allowance. Depending on means he may also qualify for the GMS (medical card)