If he cannot achieve an erection and does not ejaculate then there are a number of techniques which can be used in order to collect his sperm. The most useful is probably electro-ejaculation for which he might need an anaesthetic (or not, depending on the level of his injury). Sperm collected in this way is usually good enough to use for artificial insemination, although several ejaculations may be necessary to achieve a good sample. You will need a referral to a regional fertility centre with these facilities